Life's Greatest Miracle KEY
Teacher Notes: This program is not appropriate for younger viewers. The video is narrated by John Lithgow, shows some partial nudity and childbirth at the end. A permission form is included, I strongly recommend previewing the video before showing it to your class.
I do use this program in my senior anatomy class because it is a fantastic resource for the discussion of tissues and how the body plan is governed by the action of genes. The program shows how chromosomes pair and exchange genes, how these genes are turned on to direct the development of tissue layers. The three main tissue layers are discussed with regard to body plan (ectoderm, mesoderm, endoderm). The program also show how a single cell pinches inward during gastrulation to create the three germ layers.
Another video "From Conception to Birth" focuses on the changes in the mother's body, this one is mainly about the embryo. This video is probably more appropriate for younger viewers, as many of the scenes are cleverly edited and some things are implied rather than shown.
Note: This program may not be suitable for all viewers. | Download Permission Form
1. DNA is very good at ____________making copies of itself_____
2. Why is sexual reproduction more advantageous than cloning? _________ offspring are different from parents _________
3. How many new sperm are created every day? _______100 million _________
4. During meiosis, what happens to the matching X-shaped chromosomes? ______cling together and exchange material__________________
5. How is the egg pushed along within the Fallopian tube? ___cilia or muscular contractions __
6. To fertilize the egg, sperm encounter several obstacles, what is the last one they encounter before they penetrate the egg? ___zona ___
7. The blatocyst must do two things to survive, it must break out of the zona and what else ? _____ find a source of nutrients _____
8. Two weeks after conception, the cells organize themselve into an ____embryo_____, a process called gastrulation.
9. The three layers of cells created during gastrulation are destined for three different functions. Describe what each will become:
Bottom Layer ________digestive tract___________ Middle Layer _____muscles, bones__________ Top Layer ________nervous tissue_________
10. Genes turn on and off to make proteins. Collagen is a protein that makes tendons and bones, keratin makes hair. Hemoglobin in the blood has what job? ________carry oxygen ___________
11.An embryo with two X chromosomes will become a ___girl___. An embryo with the XY combination will become a ___boy_______.
12. Two months after fertilization, the embryo is called a _______fetus ______________
13. What structure is used to gather blood and nutrients from the mother's blood and pass it to the umbilical cord? ___ placenta___
14. At the end of pregnancy, the baby is growing myelin around its neurons. This growth increases its need for what nutrient? __ fat __
15. Why is it harder for human babies to be delivered than other animals? ___ shape of pelvis, size of head ____
Stages of Prenatal Development
While you might think of child development as something that begins during infancy, the prenatal period is also considered an important part of the developmental process. Prenatal development is a time of remarkable change that helps set the stage for future psychological development. The brain develops over the course of the prenatal period, but it will continue to go through more changes during the early years of childhood
The process of prenatal development occurs in three main stages. The first two weeks after conception are known as the germinal stage, the third through the eighth week is known as the embryonic period, and the time from the ninth week until birth is known as the fetal period.
The germinal stage begins at conception when the sperm and egg cell unite in one of the two fallopian tubes. The fertilized egg is called a zygote. Just a few hours after conception, the single-celled zygote begins making a journey down the fallopian tube to the uterus.
Cell division begins approximately 24 to 36 hours after conception. Through the process of mitosis, the zygote first divides into two cells, then into four, eight, sixteen, and so on. A significant number of zygotes never progress past this early part of cell division, with as many as half of all zygotes surviving less than two weeks.
Once the eight-cell point has been reached, the cells begin to differentiate and take on certain characteristics that will determine the type of cells they will eventually become. As the cells multiply, they will also separate into two distinctive masses: the outer cells will eventually become the placenta, while the inner cells form the embryo.
Cell division continues at a rapid rate during the approximately week-long journey from fallopian tube to uterus wall. The cells develop into what is known as a blastocyst. The blastocyst is made up of three layers, each of which develops into different structures in the body.
- Ectoderm: Skin and nervous system
- Endoderm: Digestive and respiratory systems
- Mesoderm: Muscle and skeletal systems
Finally, the blastocyst arrives at the uterus and attaches to the uterine wall, a process known as implantation. Implantation occurs when the cells nestle into the uterine lining and rupture tiny blood vessels. The connective web of blood vessels and membranes that form between them will provide nourishment for the developing being for the next nine months. Implantation is not always an automatic and sure-fire process.
Researchers estimate that approximately 60% of all natural conceptions never become properly implanted in the uterus, which results in the new life ending before the mother is ever aware she is pregnant.
When implantation is successful, hormonal changes halt the normal menstrual cycle and cause a whole host of physical changes. For some people, activities they previously enjoyed such as smoking and drinking alcohol or coffee may become less palatable, possibly part of nature’s way of protecting the growing life inside them.
At this point, the mass of cells is now known as an embryo. The beginning of the third week after conception marks the start of the embryonic period, a time when the mass of cells becomes distinct as a human. The embryonic stage plays an important role in the development of the brain.
Approximately four weeks after conception, the neural tube forms. This tube will later develop into the central nervous system including the spinal cord and brain. The neural tube begins to form along with an area known as the neural plate. The earliest signs of development of the neural tube are the emergence of two ridges that form along each side of the neural plate.
Over the next few days, more ridges form and fold inward until a hollow tube is formed. Once this tube is fully formed, cells begin to form near the center. The tube begins to close and brain vesicles form. These vesicles will eventually develop into parts of the brain, including the structures of the forebrain, midbrain, and hindbrain.
Around the fourth week, the head begins to form, quickly followed by the eyes, nose, ears, and mouth. The blood vessel that will become the heart start to pulse. During the fifth week, buds that will form the arms and legs appear.
By the eighth week of development, the embryo has all of the basic organs and parts except those of the sex organs. At this point, the embryo weighs just one gram and is about one inch in length.
By the end of the embryonic period, the basic structures of the brain and central nervous system have been established. At this point, the basic structure of the peripheral nervous system is also defined.
The production of neurons, or brain cells, begins around day 42 after conception and is mostly complete sometime around the middle of pregnancy.
As neurons form, they migrate to different areas of the brain. Once they have reached the correct location, they begin to form connections with other neural cells, establishing rudimentary neural networks.
Once cell differentiation is mostly complete, the embryo enters the next stage and becomes known as a fetus. The fetal period of prenatal develop marks more important changes in the brain. This period of development begins during the ninth week and lasts until birth. This stage is marked by amazing change and growth.
The early body systems and structures established in the embryonic stage continue to develop. The neural tube develops into the brain and spinal cord and neurons continue to form. Once these neurons have formed, they begin to migrate to their correct locations. Synapses, or the connections between neurons, also begin to develop.
Between the ninth and twelfth week of gestation (at the earliest), reflexes begin to emerge. The fetus begins to make reflexive motions with its arms and legs.
During the third month of gestation, the sex organs begin to differentiate. By the end of the month, all parts of the body will be formed. At this point, the fetus weighs around three ounces. The fetus continues to grow in both weight and length, although the majority of the physical growth occurs in the later stages of pregnancy.
The end of the third month also marks the end of the first trimester of pregnancy. During the second trimester, or months four through six, the heartbeat grows stronger and other body systems become further developed. Fingernails, hair, eyelashes, and toenails form. Perhaps most noticeably, the fetus increases about six times in size.
So what's going on inside the brain during this important period of prenatal development? The brain and central nervous system also become more responsive during the second trimester. Around 28 weeks, the brain starts to mature faster, with an activity that greatly resembles that of a sleeping newborn.
During the period from seven months until birth, the fetus continues to develop, put on weight, and prepare for life outside the womb. The lungs begin to expand and contract, preparing the muscles for breathing.
While development usually follows this normal pattern, there are times when problems with prenatal development occur. Disease, malnutrition, and other prenatal influences can have a powerful impact on how the brain develops during this critical period.
A Word From Verywell
Brain development does not end at birth. A considerable amount of brain development takes place postnatally, including growing in size and volume while changing in structure. The brain quadruples in size between birth and preschool. As children learn and have new experiences, some networks in the brain are strengthened while other connections are pruned.
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Verywell Mind uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
National Institutes of Health. What are stem cells, and why are they important?. Updated 2016.
Flaxman SM, Sherman PW. Morning sickness: A mechanism for protecting mother and embryo. Q Rev Biol. 2000;75(2):113-48. doi:10.1086/393377
13.1 The Embryologic Perspective. In: Anatomy and Physiology. Rice University.
28.3 Fetal Development. In Anatomy and Physiology. Rice University.
U.S. Department of Health and Human Services Office on Women's Health. Stages of pregnancy. Updated April 18, 2019.
Levine LE, Munsch J. Child Development: An Active Learning Approach. SAGE Publications, 2010.
Shaffer DR, Kipp K. Developmental Psychology: Childhood and Adolescence. Wadsworth, 2010.
Stiles J, Jernigan TL. The basics of brain development. Neuropsychol Rev. 2010;20(4):327-48. doi:10.1007/s11065-010-9148-4
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What you’ll learn to do: explain the main stages of prenatal development
How did you come to be who you are? From beginning as a one-cell structure to your birth, your prenatal development occurred in an orderly and delicate sequence. There are three stages of prenatal development: germinal, embryonic, and fetal. Keep in mind that this is different than the three trimesters of pregnancy. Let’s take a look at what happens to the developing baby in each of these stages.
- Differentiate between development during the germinal, embryonic, and fetal periods
- Examine risks to prenatal development posed by exposure to teratogens
- Explain potential complications of pregnancy and delivery
“The body of the unborn baby is more complex than ours. The preborn baby has several extra parts to his body which he needs only so long as he lives inside his mother. He has his own space capsule, the amniotic sac. He has his own lifeline, the umbilical cord, and he has his own root system, the placenta. These all belong to the baby himself, not to his mother. They are all developed from his original cell.”
Periods of Prenatal Development
Let’s take a look at some of the changes that take place during each of the three periods of prenatal development: the germinal period, the embryonic period, and the fetal period.
The Germinal Period (Weeks 1-2)
Figure 1. Sperm and Ovum at Conception
Conception occurs when a sperm fertilizes an egg and forms a zygote, which begins as a one-cell structure. The mother and father’s DNA is passed on to the child at the moment of conception. The genetic makeup and sex of the baby are set at this point. The germinal period (about 14 days in length) lasts from conception to implantation of the zygote (fertilized egg) in the lining of the uterus.
During the first week after conception, the zygote divides and multiplies, going from a one-cell structure to two cells, then four cells, then eight cells, and so on. The process of cell division is called mitosis. After the fourth division, differentiation of the cells begins to occur as well. Differentiated cells become more specialized, forming different organs and body parts. After 5 days of mitosis, there are 100 cells, and after 9 months there are billions of cells. Mitosis is a fragile process, and fewer than one-half of all zygotes survive beyond the first two weeks (Hall, 2004).
After the zygote divides for about 7–10 days and has 150 cells, it travels down the fallopian tubes and implants itself in the lining of the uterus. It’s estimated that about 60 percent of natural conceptions fail to implant in the uterus. The rate is higher for in vitro conceptions. Once the zygote attaches to the uterus, the next stage begins.
The Embryonic Period (Weeks 3-8)
Figure 2. Human Embryo
The embryonic period begins once the zygote is implanted in the uterine wall. It lasts from the third through the eighth week after conception. Upon implantation, this multi-cellular organism is called an embryo. Now blood vessels grow, forming the placenta. The placenta is a structure connected to the uterus that provides nourishment and oxygen from the mother to the developing embryo via the umbilical cord.
During this period, cells continue to differentiate. Basic structures of the embryo start to develop into areas that will become the head, chest, and abdomen. During the embryonic stage, the heart begins to beat and organs form and begin to function. At 22 days after conception, the neural tube forms along the back of the embryo, developing into the spinal cord and brain.
Growth during prenatal development occurs in two major directions: from head to tail (cephalocaudal development) and from the midline outward (proximodistal development). This means that those structures nearest the head develop before those nearest the feet and those structures nearest the torso develop before those away from the center of the body (such as hands and fingers).
The head develops in the fourth week and the precursor to the heart begins to pulse. In the early stages of the embryonic period, gills and a tail are apparent. But by the end of this stage, they disappear and the organism takes on a more human appearance. The embryo is approximately 1 inch in length and weighs about 4 grams at the end of this period. The embryo can move and respond to touch at this time.
About 20 percent of organisms fail during the embryonic period, usually due to gross chromosomal abnormalities. As in the case of the germinal period, often the mother does not yet know that she is pregnant. It is during this stage that the major structures of the body are taking form making the embryonic period the time when the organism is most vulnerable to the greatest amount of damage if exposed to harmful substances. Potential mothers are not often aware of the risks they introduce to the developing child during this time.
The Fetal Period (Weeks 9-40)
Figure 3. A fetus at 10 weeks of development.
When the organism is about nine weeks old, the embryo is called a fetus. At this stage, the fetus is about the size of a kidney bean and begins to take on the recognizable form of a human being as the “tail” begins to disappear.
From 9–12 weeks, the sex organs begin to differentiate. By the 12th week, the fetus has all its body parts including external genitalia. In the following weeks, the fetus will develop hair, nails, teeth and the excretory and digestive systems will continue to develop. At the end of the 12th week, the fetus is about 3 inches long and weighs about 28 grams.
At about 16 weeks, the fetus is approximately 4.5 inches long. Fingers and toes are fully developed, and fingerprints are visible. During the 4-6th months, the eyes become more sensitive to light and hearing develops. The respiratory system continues to develop. Reflexes such as sucking, swallowing and hiccuping develop during the 5th month. Cycles of sleep and wakefulness are present at that time as well. Throughout the fetal stage, the brain continues to grow and develop, nearly doubling in size from weeks 16 to 28. The majority of the neurons in the brain have developed by 24 weeks although they are still rudimentary and the glial or nurse cells that support neurons continue to grow. At 24 weeks the fetus can feel pain (Royal College of Obstetricians and Gynecologists, 1997).
The first chance of survival outside the womb, known as the age of viability is reached at about 22 to 26 weeks (Moore & Persaud, 1998). By the time the fetus reaches the sixth month of development (24 weeks), it weighs up to 1.4 pounds. The hearing has developed, so the fetus can respond to sounds. The internal organs, such as the lungs, heart, stomach, and intestines, have formed enough that a fetus born prematurely at this point has a chance to survive outside of the mother’s womb.
Between the 7th and 9th months, the fetus is primarily preparing for birth. It is exercising its muscles, its lungs begin to expand and contract. It is developing fat layers under the skin. The fetus gains about 5 pounds and 7 inches during this last trimester of pregnancy which includes a layer of fat gained during the 8th month. This layer of fat serves as insulation and helps the baby regulate body temperature after birth.
Around 36 weeks, the fetus is almost ready for birth. It weighs about 6 pounds and is about 18.5 inches long, and by week 37 all of the fetus’s organ systems are developed enough that it could survive outside the mother’s uterus without many of the risks associated with premature birth. The fetus continues to gain weight and grow in length until approximately 40 weeks. By then, the fetus has very little room to move around and birth becomes imminent.
Figure 4. During the fetal stage, the baby’s brain develops and the body adds size and weight until the fetus reaches full-term development.
This video explains many of the developmental milestones and changes that happen during each month of development for the embryo and fetus.
Good prenatal care is essential. The developing child is most at risk for some of the most severe problems during the first three months of development. Unfortunately, this is a time at which most mothers are unaware that they are pregnant. It is estimated that 10% of all birth defects are caused by a prenatal exposure or teratogen. Teratogens are factors that can contribute to birth defects which include some maternal diseases, drugs, alcohol, and stress. These exposures can also include environmental and occupational exposures. Today, we know many of the factors that can jeopardize the health of the developing child. Teratogen-caused birth defects are potentially preventable.
The study of factors that contribute to birth defects is called teratology. Teratogens are usually discovered after an increased prevalence of a particular birth defect. For example, in the early 1960’s, a drug known as thalidomide was used to treat morning sickness. Exposure of the fetus during this early stage of development resulted in cases of phocomelia, a congenital malformation in which the hands and feet are attached to abbreviated arms and legs.
A Look at Some Teratogens
Figure 5. Some distinguishing characteristics of fetal alcohol spectrum disorders include more narrow eye openings, A smooth philtrum, meaning a smooth area between the upper lip and the nose, and a thin upper lip.
One of the most commonly used teratogens is alcohol. Because half of all pregnancies in the United States are unplanned, it is recommended that women of child-bearing age take great caution against drinking alcohol when not using birth control and when pregnant (Surgeon General’s Advisory on Alcohol Use During Pregnancy, 2005). Alcohol consumption, particularly during the second month of prenatal development, but at any point during pregnancy, may lead to neurocognitive and behavioral difficulties that can last a lifetime.
There is no acceptable safe limit for alcohol use during pregnancy, but binge drinking (5 or more drinks on a single occasion) or having 7 or more drinks during a single week places a child at particularly high risk. In extreme cases, alcohol consumption can lead to fetal death, but more frequently it can result in fetal alcohol spectrum disorders (FASD). This terminology is now used when looking at the effects of exposure and replaces the term fetal alcohol syndrome. It is preferred because it recognizes that symptoms occur on a spectrum and that all individuals do not have the same characteristics. Children with FASD share certain physical features such as flattened noses, small eye openings, small heads, intellectual developmental delays, and behavioral problems. Those with FASD are more at risk for lifelong problems such as criminal behavior, psychiatric problems, and unemployment (CDC, 2006).
The terms alcohol-related neurological disorder (ARND) and alcohol-related birth defects (ARBD) have replaced the term Fetal Alcohol Effects to refer to those with less extreme symptoms of FASD. ARBD include kidney, bone and heart problems.
Several medical experts debunk common myths about the safety of drinking alcohol during pregnancy.
Smoking is also considered a teratogen because nicotine travels through the placenta to the fetus. When the mother smokes, the developing baby experiences a reduction in blood oxygen levels. Tobacco use during pregnancy has been associated with low birth weight, placenta previa, birth defects, preterm delivery, fetal growth restriction, and sudden infant death syndrome. Smoking in the month before getting pregnant and throughout pregnancy increases the chances of these risks. Quitting smoking before getting pregnant is best. However, for women who are already pregnant, quitting as early as possible can still help protect against some health problems for the mother and baby.
Prescription, over-the-counter, or recreational drugs can have serious teratogenic effects. In general, if medication is required, the lowest dose possible should be used. Combination drug therapies and first trimester exposures should be avoided. Almost three percent of pregnant women use illicit drugs such as marijuana, cocaine, Ecstasy and other amphetamines, and heroin. These drugs can cause low birth-weight, withdrawal symptoms, birth defects, or learning or behavioral problems. Babies born with a heroin addiction need heroin just like an adult addict. The child will need to be gradually weaned from the heroin under medical supervision; otherwise, the child could have seizures and die.
Environmental chemicals can include an exposure to a wide array of agents including pollution, organic mercury compounds, herbicides, and industrial solvents. Some environmental pollutants of major concern include lead poisoning, which is connected with low birth weight and slowed neurological development. Children who live in older housing in which lead-based paints have been used have been known to eat peeling paint chips thus being exposed to lead. The chemicals in certain herbicides are also potentially damaging. Radiation is another environmental hazard that a pregnant woman must be aware of. If a mother is exposed to radiation, particularly during the first three months of pregnancy, the child may suffer some congenital deformities. There is also an increased risk of miscarriage and stillbirth. Mercury leads to physical deformities and intellectual disabilities (Dietrich, 1999).
Sexually Transmitted Infections
Sexually transmitted infections (STIs) can complicate pregnancy and may have serious effects on both the mother and the developing baby. Most prenatal care today includes testing for STIs, and early detection is important. STIs, such as chlamydia, gonorrhea, syphilis, trichomoniasis and bacterial vaginosis can all be treated and cured with antibiotics that are safe to take during pregnancy. STIs that are caused by viruses, like genital herpes, hepatitis B, or HIV cannot be cured. However, in some cases these infections can be treated with antiviral medications or other preventive measures can be taken to reduce the risk of passing the infection to the baby.
Maternal illnesses increase the chance that a baby will be born with a birth defect or have a chronic health problem. Some of the diseases that are known to potentially have an adverse effect on the fetus include: diabetes, cytomegalovirus, toxoplasmosis, Rubella, varicella, hypothyroidism, and Strep B. If the mother contracts Rubella during the first three months of pregnancy, damage can occur in the eyes, ears, heart, or brain of the unborn child. On a positive note, Rubella has been nearly eliminated in the industrial world due to the vaccine created in 1969. Diagnosing these diseases early and receiving appropriate medical care can help improve the outcomes. Routine prenatal care now includes screening for gestational diabetes and Strep B.
Stress represents the effects of any factor able to threaten the homeostasis of an organism; these either real or perceived threats are referred to as the “stressors” and comprise a long list of potentially adverse factors, which can be emotional or physical. Because of a link in blood supply between a mother and fetus, it has been found that stress can leave lasting effects on a developing fetus, even before a child is born. The best-studied outcomes of fetal exposure to maternal prenatal stress are preterm birth and low birth weight. Maternal prenatal stress is also considered responsible for a variety of changes of the child’s brain, and a risk factor for conditions such as behavioral problems, learning disorders, high levels of anxiety, attention deficit hyperactivity disorder, autism, and schizophrenia. Furthermore, maternal prenatal stress has been associated with a higher risk for a variety of immune and metabolic changes in the child such as asthma, allergic disorders, cardiovascular diseases, hypertension, hyperlipidemia, diabetes, and obesity.
Factors influencing prenatal risks
There are several considerations in determining the type and amount of damage that might result from exposure to a particular teratogen (Berger, 2004). These include:
- The timing of the exposure: Structures in the body are vulnerable to the most severe damage when they are forming. If a substance is introduced during a particular structure’s critical period (time of development), the damage to that structure may be greater. For example, the ears and arms reach their critical periods at about 6 weeks after conception. If a mother exposes the embryo to certain substances during this period, the arms and ears may be malformed.
- The amount of exposure: Some substances are not harmful unless the amounts reach a certain level. The critical level depends in part on the size and metabolism of the mother.
- Genetics: Genetic make-up also plays a role on the impact a particular teratogen might have on the child. This is suggested by fraternal twin studies who are exposed to the same prenatal environment, yet do not experience the same teratogenic effects. The genetic make-up of the mother can also have an effect; some mothers may be more resistant to teratogenic effects than others.
- Being male or female: Males are more likely to experience damage due to teratogens than are females. It is believed that the Y chromosome, which contains fewer genes than the X, may have an impact.
Figure 6. Critical Periods of Prenatal Development. This image summarizes the three developmental periods in prenatal development. The blue images indicate where major development is happening and the aqua indicate where refinement is happening. As shown, the majority of organs are particularly susceptible during the embryonic period. The central nervous system still continues to develop in major ways through the fetal period as well.
Interactive: reducing the risk
Did you know that pregnant women can improve outcomes for themselves and their babies through a balanced diet and adequate exercise? Click through this interactive to learn more about the importance of maternal health.
Complications of Pregnancy and Delivery
Complications of Pregnancy and Delivery
Figure 7. Pregnancy affects women in different ways; some notice few adverse side effects, while others feel high levels of discomfort, or develop more serious complications.
There are a number of common side effects of pregnancy. Not everyone experiences all of these nor do women experience them to the same degree. And although they are considered “minor” these problems are potentially very uncomfortable. These side effects include nausea (particularly during the first 3-4 months of pregnancy as a result of higher levels of estrogen in the system), heartburn, gas, hemorrhoids, backache, leg cramps, insomnia, constipation, shortness of breath or varicose veins (as a result of carrying a heavy load on the abdomen). What is the cure? Delivery!
The following are some serious complications of pregnancy which can pose health risks to mother and child and that often require special care.
- Gestational diabetes is when a woman without diabetes develops high blood sugar levels during pregnancy.
- Hyperemesis gravidarum is the presence of severe and persistent vomiting, causing dehydration and weight loss. It is more severe than the more common morning sickness.
- Preeclampsia is gestational hypertension. Severe preeclampsia involves blood pressure over 160/110 with additional signs. Eclampsia is seizures in a pre-eclamptic patient.
- Deep vein thrombosis is the formation of a blood clot in a deep vein, most commonly in the legs.
- A pregnant woman is more susceptible to infections. This increased risk is caused by an increased immune tolerance in pregnancy to prevent an immune reaction against the fetus.
- Peripartum cardiomyopathy is a decrease in heart function which occurs in the last month of pregnancy, or up to six months post-pregnancy.
Maternal mortality is unacceptably high. About 830 women die from pregnancy or childbirth-related complications around the world every day. It was estimated that in 2015, roughly 303,000 women died during and following pregnancy and childbirth. Almost all of these deaths occurred in low-resource settings, and most could have been prevented. The high number of maternal deaths in some areas of the world reflects inequities in access to health services and highlights the gap between rich and poor. Almost all maternal deaths (99%) occur in developing countries. More than half of these deaths occur in sub-Saharan Africa and almost one third occur in South Asia.
Almost all maternal deaths can be prevented, as evidenced by the huge disparities found between the richest and poorest countries. The lifetime risk of maternal death in high-income countries is 1 in 3,300, compared to 1 in 41 in low-income.
Figure 8. This graph shows declining maternal mortality rates, as measured as the number of deaths per 100,000 live births. in 1990, 903 out of 100,000 live births resulted in death in the least developed countries, but that number has improved to 436 out of 100,000 births in 2015. Globally, there were 216 deaths for every 100,000 live births in 2015. Source: UNICEF, https://data.unicef.org/topic/maternal-health/maternal-mortality/.
Even though maternal mortality in the United States is relatively rare today because of advanced in medical care, it is still an issue that needs to be addressed. The number of reported pregnancy-related deaths in the United States steadily increased from 7.2 deaths per 100,000 live births in 1987 to 18.0 deaths per 100,000 live births in 2014. The Centers for Disease Control and Prevention define a pregnancy-related death as the death of a woman while pregnant or within 1 year of the end of a pregnancy–regardless of the outcome, duration, or site of the pregnancy–from any cause related to or aggravated by the pregnancy or its management, but not from accidental or incidental causes. The reasons for the overall increase in pregnancy-related mortality are unclear. What do you think are some reasons for this surprising increase in the United States? What can be done to change this statistic?
Watch It: Maternal Mortality in the united States
In the United States, black women are disproportionately more likely to die from complications related to pregnancy or childbirth than any other race; they are three or four times more likely than white women to die due to pregnancy-related death and are more likely to receive worse maternal care. Black women from higher income groups and with advanced education levels also have heightened risks—even tennis superstar Serena Williams had near-deadly complications during the birth of her daughter, Olympia. Why is this the case in our modern world? Watch this video to learn more:
The data below shows percentages of the causes of pregnancy-related deaths in the United States during 2011–2014:
- Cardiovascular diseases, 15.2%.
- Non-cardiovascular diseases, 14.7%.
- Infection or sepsis, 12.8%.
- Hemorrhage, 11.5%.
- Cardiomyopathy, 10.3%.
- Thrombotic pulmonary embolism, 9.1%.
- Cerebrovascular accidents, 7.4%.
- Hypertensive disorders of pregnancy, 6.8%.
- Amniotic fluid embolism, 5.5%.
- Anesthesia complications, 0.3%.
The cause of death is unknown for 6.5% of all 2011–2014 pregnancy-related deaths.
Spontaneous abortion is experienced in an estimated 20-40 percent of undiagnosed pregnancies and in another 10 percent of diagnosed pregnancies. Usually, the body aborts due to chromosomal abnormalities and this typically happens before the 12th week of pregnancy. Cramping and bleeding result and normal periods return after several months. Some women are more likely to have repeated miscarriages due to chromosomal, amniotic, or hormonal problems; but miscarriage can also be a result of defective sperm (Carroll et al., 2003).
- a multi-celled organism between two and eight weeks after fertilization
- fetal alcohol spectrum disorders:
- a group of abnormalities in babies born to mothers who consume alcohol during pregnancy
- an unborn human baby from nine weeks after conception until birth
- the process of cell division
- a structure connected to the uterus that provides nourishment and oxygen from the mother to the developing embryo via the umbilical cord
- pregnancy-related death:
- the death of a woman while pregnant or within 1 year of the end of a pregnancy from any cause related to or aggravated by the pregnancy
- any agent which can cause a birth defect
- a one-cell structure that is created when a sperm and egg merge
7.1 Conception and Prenatal Development
Chapter 7. Growing and Developing
- Review the stages of prenatal development.
- Explain how the developing embryo and fetus may be harmed by the presence of teratogens and describe what a mother can do to reduce her risk.
Conceptionoccurs when an egg from the mother is fertilized by a sperm from the father. In humans, the conception process begins with ovulation, when an ovum, or egg (the largest cell in the human body), which has been stored in one of the mother’s two ovaries, matures and is released into the fallopian tube. Ovulation occurs about halfway through the woman’s menstrual cycle and is aided by the release of a complex combination of hormones. In addition to helping the egg mature, the hormones also cause the lining of the uterus to grow thicker and more suitable for implantation of a fertilized egg.
If the woman has had sexual intercourse within one or two days of the egg’s maturation, one of the up to 500 million sperm deposited by the man’s ejaculation, which are travelling up the fallopian tube, may fertilize the egg. Although few of the sperm are able to make the long journey, some of the strongest swimmers succeed in meeting the egg. As the sperm reach the egg in the fallopian tube, they release enzymes that attack the outer jellylike protective coating of the egg, each trying to be the first to enter. As soon as one of the millions of sperm enters the egg’s coating, the egg immediately responds by both blocking out all other challengers and at the same time pulling in the single successful sperm.
Within several hours of conception, half of the 23 chromosomes from the egg and half of the 23 chromosomes from the sperm fuse together, creating a zygote — a fertilized ovum. The zygote continues to travel down the fallopian tube to the uterus. Although the uterus is only about four inches away in the woman’s body, the zygote’s journey is nevertheless substantial for a microscopic organism, and fewer than half of zygotes survive beyond this earliest stage of life. If the zygote is still viable when it completes the journey, it will attach itself to the wall of the uterus, but if it is not, it will be flushed out in the woman’s menstrual flow. During this time, the cells in the zygote continue to divide: the original two cells become four, those four become eight, and so on, until there are thousands (and eventually trillions) of cells. Soon the cells begin to differentiate, each taking on a separate function. The earliest differentiation is between the cells on the inside of the zygote, which will begin to form the developing human being, and the cells on the outside, which will form the protective environment that will provide support for the new life throughout the pregnancy.
Once the zygote attaches to the wall of the uterus, it is known as theembryo. During the embryonic phase, which will last for the next six weeks, the major internal and external organs are formed, each beginning at the microscopic level, with only a few cells. The changes in the embryo’s appearance will continue rapidly from this point until birth.
While the inner layer of embryonic cells is busy forming the embryo itself, the outer layer is forming the surrounding protective environment that will help the embryo survive the pregnancy. This environment consists of three major structures: The amniotic sac is the fluid-filled reservoir in which the embryo (soon to be known as a fetus) will live until birth, and which acts as both a cushion against outside pressure and as a temperature regulator. The placenta is an organ that allows the exchange of nutrients between the embryo and the mother, while at the same time filtering out harmful material. The filtering occurs through a thin membrane that separates the mother’s blood from the blood of the fetus, allowing them to share only the material that is able to pass through the filter. Finally, the umbilical cordlinks the embryo directly to the placenta and transfers all material to the fetus. Thus the placenta and the umbilical cord protect the fetus from many foreign agents in the mother’s system that might otherwise pose a threat.
Beginning in the ninth week after conception, the embryo becomes a fetus. The defining characteristic of the fetal stage is growth. All the major aspects of the growing organism have been formed in the embryonic phase, and now the fetus has approximately six months to go from weighing less than an ounce to weighing an average of six to eight pounds. That’s quite a growth spurt.
The fetus begins to take on many of the characteristics of a human being, including moving (by the third month the fetus is able to curl and open its fingers, form fists, and wiggle its toes), sleeping, as well as early forms of swallowing and breathing. The fetus begins to develop its senses, becoming able to distinguish tastes and respond to sounds. Research has found that the fetus even develops some initial preferences. A newborn prefers the mother’s voice to that of a stranger, the languages heard in the womb over other languages (DeCasper & Fifer, 1980; Moon, Cooper, & Fifer, 1993), and even the kinds of foods that the mother ate during the pregnancy (Mennella, Jagnow, & Beauchamp, 2001). By the end of the third month of pregnancy, the sexual organs are visible.
How the Environment Can Affect the Vulnerable Fetus
Prenatal development is a complicated process and may not always go as planned. About 45% of pregnancies result in a miscarriage, often without the mother ever being aware it has occurred (Moore & Persaud, 1993). Although the amniotic sac and the placenta are designed to protect the embryo, substances that can harm the fetus, known as teratogens, may nevertheless cause problems. Teratogens include general environmental factors, such as air pollution and radiation, but also the cigarettes, alcohol, and drugs that the mother may use. Teratogens do not always harm the fetus, but they are more likely to do so when they occur in larger amounts, for longer time periods, and during the more sensitive phases, as when the fetus is growing most rapidly. The most vulnerable period for many of the fetal organs is very early in the pregnancy — before the mother even knows she is pregnant.
Harmful substances that the mother ingests may harm the child. Cigarette smoking, for example, reduces the blood oxygen for both the mother and child and can cause a fetus to be born severely underweight. Another serious threat is fetal alcohol syndrome (FAS), a condition caused by maternal alcohol drinking that can lead to numerous detrimental developmental effects, including limb and facial abnormalities, genital anomalies, and mental retardation. Each year in Canada, it is estimated that nine babies in every 1,000 are born with fetal alcohol spectrum disorder (FASD), and it is considered one of the leading causes of retardation in the world today (Health Canada, 2006; Niccols, 1994). Because there is no known safe level of alcohol consumption for a pregnant woman, the Public Health Agency of Canada (2011) states that there is no safe amount or safe time to drink alcohol during pregnancy. Therefore, the best approach for expectant mothers is to avoid alcohol completely. Maternal drug abuse is also of major concern and is considered one of the greatest risk factors facing unborn children.
The environment in which the mother is living also has a major impact on infant development (Duncan & Brooks-Gunn, 2000; Haber & Toro, 2004). Children born into homelessness or poverty are more likely to have mothers who are malnourished, who suffer from domestic violence, stress, and other psychological problems, and who smoke or abuse drugs. And children born into poverty are also more likely to be exposed to teratogens. Poverty’s impact may also amplify other issues, creating substantial problems for healthy child development (Evans & English, 2002; Gunnar & Quevedo, 2007).
Mothers normally receive genetic and blood tests during the first months of pregnancy to determine the health of the embryo or fetus. They may undergo sonogram, ultrasound, amniocentesis, or other testing (Figure 7.1). The screenings detect potential birth defects, including neural tube defects, chromosomal abnormalities (such as Down syndrome), genetic diseases, and other potentially dangerous conditions. Early diagnosis of prenatal problems can allow medical treatment to improve the health of the fetus.
- Development begins at the moment of conception, when the sperm from the father merges with the egg from the mother.
- Within a span of nine months, development progresses from a single cell into a zygote and then into an embryo and fetus.
- The fetus is connected to the mother through the umbilical cord and the placenta, which allow the fetus and mother to exchange nourishment and waste. The fetus is protected by the amniotic sac.
- The embryo and fetus are vulnerable and may be harmed by the presence of teratogens.
- Smoking, alcohol use, and drug use are all likely to be harmful to the developing embryo or fetus, and the mother should entirely refrain from these behaviours during pregnancy or if she expects to become pregnant.
- Environmental factors, especially homelessness and poverty, have a substantial negative effect on healthy child development.
Exercises and Critical Thinking
- What behaviours must a woman avoid engaging in when she decides to try to become pregnant, or when she finds out she is pregnant? Do you think the ability of a mother to engage in healthy behaviours should influence her choice to have a child?
- Given the negative effects of poverty on human development, what steps do you think societies should take to try to reduce poverty?
DeCasper, A. J., & Fifer, W. P. (1980). Of human bonding: Newborns prefer their mothers’ voices. Science, 208, 1174–1176.
Duncan, G., & Brooks-Gunn, J. (2000). Family poverty, welfare reform, and child development. Child Development, 71(1), 188–196.
Evans, G. W., & English, K. (2002). The environment of poverty: Multiple stressor exposure, psychophysiological stress, and socio-emotional adjustment. Child Development, 73(4), 1238–1248.
Gunnar, M., & Quevedo, K. (2007). The neurobiology of stress and development. Annual Review of Psychology, 58, 145–173.
Haber, M., & Toro, P. (2004). Homelessness among families, children, and adolescents: An ecological-developmental perspective. Clinical Child and Family Psychology Review, 7(3), 123–164.
Health Canada. (2006). It’s your health: Fetal alcohol spectrum disorder [PDF]. Retrieved June 2014 from http://www.hc-sc.gc.ca/hl-vs/alt_formats/pacrb-dgapcr/pdf/iyh-vsv/diseases-maladies/fasd-etcaf-eng.pdf
Mennella, J. A., Jagnow, C. P., & Beauchamp, G. K. (2001). Prenatal and postnatal flavor learning by human infants. Pediatrics, 107(6), e88.
Moon, C., Cooper, R. P., & Fifer, W. P. (1993). Two-day-olds prefer their native language. Infant Behavior & Development, 16, 495–500.
Moore, K., & Persaud, T. (1993). The developing human: Clinically oriented embryology (5th ed.). Philadelphia, PA: Saunders.
Niccols, G. A. (1994). Fetal alcohol syndrome: Implications for psychologists. Clinical Psychology Review, 14, 91–111.
Public Health Agency of Canada. (2011). The healthy pregnancy guide. Retrieved May 10, 2014 from http://www.phac-aspc.gc.ca/hp-gs/guide/index-eng.php
Figure 7.1: “Leipzig, Universitätsklinik, Untersuchung” by Grubitzsch (http://en.wikipedia.org/wiki/File:Bundesarchiv_Bild_183-1990-0417-001,_Leipzig,_Universit%C3%A4tsklinik,_Untersuchung.jpg) is licensed under CC BY-SA 3.0 DE (http://creativecommons.org/licenses/by-sa/3.0/de/deed.en).
Key answer birth conception from worksheet to
24.5. Human Pregnancy and Birth
Chapter 24. Animal Reproduction and Development
By the end of this section, you will be able to:
- Explain fetal development during the three trimesters of gestation
- Describe labor and delivery
- Compare the efficacy and duration of various types of contraception
- Discuss causes of infertility and the therapeutic options available
Pregnancy begins with the fertilization of an egg and continues through to the birth of the individual. The length of time of gestation varies among animals, but is very similar among the great apes: human gestation is 266 days, while chimpanzee gestation is 237 days, a gorilla’s is 257 days, and orangutan gestation is 260 days long. The fox has a 57-day gestation. Dogs and cats have similar gestations averaging 60 days. The longest gestation for a land mammal is an African elephant at 640 days. The longest gestations among marine mammals are the beluga and sperm whales at 460 days.
Twenty-four hours before fertilization, the egg has finished meiosis and becomes a mature oocyte. When fertilized (at conception) the egg becomes known as a zygote. The zygote travels through the oviduct to the uterus (Figure 24.18). The developing embryo must implant into the wall of the uterus within seven days, or it will deteriorate and die. The outer layers of the zygote (blastocyst) grow into the endometrium by digesting the endometrial cells, and wound healing of the endometrium closes up the blastocyst into the tissue. Another layer of the blastocyst, the chorion, begins releasing a hormone called human beta chorionic gonadotropin (β-HCG) which makes its way to the corpus luteum and keeps that structure active. This ensures adequate levels of progesterone that will maintain the endometrium of the uterus for the support of the developing embryo. Pregnancy tests determine the level of β-HCG in urine or serum. If the hormone is present, the test is positive.
The gestation period is divided into three equal periods or trimesters. During the first two to four weeks of the first trimester, nutrition and waste are handled by the endometrial lining through diffusion. As the trimester progresses, the outer layer of the embryo begins to merge with the endometrium, and the placenta forms. This organ takes over the nutrient and waste requirements of the embryo and fetus, with the mother’s blood passing nutrients to the placenta and removing waste from it. Chemicals from the fetus, such as bilirubin, are processed by the mother’s liver for elimination. Some of the mother’s immunoglobulins will pass through the placenta, providing passive immunity against some potential infections.
Internal organs and body structures begin to develop during the first trimester. By five weeks, limb buds, eyes, the heart, and liver have been basically formed. By eight weeks, the term fetus applies, and the body is essentially formed, as shown in Figure 24.19. The individual is about five centimeters (two inches) in length and many of the organs, such as the lungs and liver, are not yet functioning. Exposure to any toxins is especially dangerous during the first trimester, as all of the body’s organs and structures are going through initial development. Anything that affects that development can have a severe effect on the fetus’ survival.
During the second trimester, the fetus grows to about 30 cm (12 inches), as shown in Figure 24.20. It becomes active and the mother usually feels the first movements. All organs and structures continue to develop. The placenta has taken over the functions of nutrition and waste and the production of estrogen and progesterone from the corpus luteum, which has degenerated. The placenta will continue functioning up through the delivery of the baby.
During the third trimester, the fetus grows to 3 to 4 kg (6 ½ -8 ½ lbs.) and about 50 cm (19-20 inches) long, as illustrated in Figure 24.21. This is the period of the most rapid growth during the pregnancy. Organ development continues to birth (and some systems, such as the nervous system and liver, continue to develop after birth). The mother will be at her most uncomfortable during this trimester. She may urinate frequently due to pressure on the bladder from the fetus. There may also be intestinal blockage and circulatory problems, especially in her legs. Clots may form in her legs due to pressure from the fetus on returning veins as they enter the abdominal cavity.
Concept in Action
Visit this site to see the stages of human fetal development.
Labor is the physical efforts of expulsion of the fetus and the placenta from the uterus during birth (parturition). Toward the end of the third trimester, estrogen causes receptors on the uterine wall to develop and bind the hormone oxytocin. At this time, the baby reorients, facing forward and down with the back or crown of the head engaging the cervix (uterine opening). This causes the cervix to stretch and nerve impulses are sent to the hypothalamus, which signals for the release of oxytocin from the posterior pituitary. The oxytocin causes the smooth muscle in the uterine wall to contract. At the same time, the placenta releases prostaglandins into the uterus, increasing the contractions. A positive feedback relay occurs between the uterus, hypothalamus, and the posterior pituitary to assure an adequate supply of oxytocin. As more smooth muscle cells are recruited, the contractions increase in intensity and force.
There are three stages to labor. During stage one, the cervix thins and dilates. This is necessary for the baby and placenta to be expelled during birth. The cervix will eventually dilate to about 10 cm. During stage two, the baby is expelled from the uterus. The uterus contracts and the mother pushes as she compresses her abdominal muscles to aid the delivery. The last stage is the passage of the placenta after the baby has been born and the organ has completely disengaged from the uterine wall. If labor should stop before stage two is reached, synthetic oxytocin, known as Pitocin, can be administered to restart and maintain labor.
An alternative to labor and delivery is the surgical delivery of the baby through a procedure called a Caesarian section. This is major abdominal surgery and can lead to post-surgical complications for the mother, but in some cases it may be the only way to safely deliver the baby.
The mother’s mammary glands go through changes during the third trimester to prepare for lactation and breastfeeding. When the baby begins suckling at the breast, signals are sent to the hypothalamus causing the release of prolactin from the anterior pituitary. Prolactin causes the mammary glands to produce milk. Oxytocin is also released, promoting the release of the milk. The milk contains nutrients for the baby’s development and growth as well as immunoglobulins to protect the child from bacterial and viral infections.
Contraception and Birth Control
The prevention of a pregnancy comes under the terms contraception or birth control. Strictly speaking, contraception refers to preventing the sperm and egg from joining. Both terms are, however, frequently used interchangeably.
|Method||Examples||Failure Rate in Typical Use Over 12 Months|
|Barrier||male condom, female condom, sponge, cervical cap, diaphragm, spermicides||15 to 24%|
|Hormonal||oral, patch, vaginal ring||8%|
|implant||less than 1%|
|Other||natural family planning||12 to 25%|
|sterilization||less than 1%|
Table 24.3 lists common methods of contraception. The failure rates listed are not the ideal rates that could be realized, but the typical rates that occur. A failure rate is the number of pregnancies resulting from the method’s use over a twelve-month period. Barrier methods, such as condoms, cervical caps, and diaphragms, block sperm from entering the uterus, preventing fertilization. Spermicides are chemicals that are placed in the vagina that kill sperm. Sponges, which are saturated with spermicides, are placed in the vagina at the cervical opening. Combinations of spermicidal chemicals and barrier methods achieve lower failure rates than do the methods when used separately.
Nearly a quarter of the couples using barrier methods, natural family planning, or withdrawal can expect a failure of the method. Natural family planning is based on the monitoring of the menstrual cycle and having intercourse only during times when the egg is not available. A woman’s body temperature may rise a degree Celsius at ovulation and the cervical mucus may increase in volume and become more pliable. These changes give a general indication of when intercourse is more or less likely to result in fertilization. Withdrawal involves the removal of the penis from the vagina during intercourse, before ejaculation occurs. This is a risky method with a high failure rate due to the possible presence of sperm in the bulbourethral gland’s secretion, which may enter the vagina prior to removing the penis.
Hormonal methods use synthetic progesterone (sometimes in combination with estrogen), to inhibit the hypothalamus from releasing FSH or LH, and thus prevent an egg from being available for fertilization. The method of administering the hormone affects failure rate. The most reliable method, with a failure rate of less than 1 percent, is the implantation of the hormone under the skin. The same rate can be achieved through the sterilization procedures of vasectomy in the man or of tubal ligation in the woman, or by using an intrauterine device (IUD). IUDs are inserted into the uterus and establish an inflammatory condition that prevents fertilized eggs from implanting into the uterine wall.
Compliance with the contraceptive method is a strong contributor to the success or failure rate of any particular method. The only method that is completely effective at preventing conception is abstinence. The choice of contraceptive method depends on the goals of the woman or couple. Tubal ligation and vasectomy are considered permanent prevention, while other methods are reversible and provide short-term contraception.
Termination of an existing pregnancy can be spontaneous or voluntary. Spontaneous termination is a miscarriage and usually occurs very early in the pregnancy, usually within the first few weeks. This occurs when the fetus cannot develop properly and the gestation is naturally terminated. Voluntary termination of a pregnancy is an abortion. Laws regulating abortion vary between states and tend to view fetal viability as the criteria for allowing or preventing the procedure.
Infertility is the inability to conceive a child or carry a child to birth. About 75 percent of causes of infertility can be identified; these include diseases, such as sexually transmitted diseases that can cause scarring of the reproductive tubes in either men or women, or developmental problems frequently related to abnormal hormone levels in one of the individuals. Inadequate nutrition, especially starvation, can delay menstruation. Stress can also lead to infertility. Short-term stress can affect hormone levels, while long-term stress can delay puberty and cause less frequent menstrual cycles. Other factors that affect fertility include toxins (such as cadmium), tobacco smoking, marijuana use, gonadal injuries, and aging.
If infertility is identified, several assisted reproductive technologies (ART) are available to aid conception. A common type of ART is in vitro fertilization (IVF) where an egg and sperm are combined outside the body and then placed in the uterus. Eggs are obtained from the woman after extensive hormonal treatments that prepare mature eggs for fertilization and prepare the uterus for implantation of the fertilized egg. Sperm are obtained from the man and they are combined with the eggs and supported through several cell divisions to ensure viability of the zygotes. When the embryos have reached the eight-cell stage, one or more is implanted into the woman’s uterus. If fertilization is not accomplished by simple IVF, a procedure that injects the sperm into an egg can be used. This is called intracytoplasmic sperm injection (ICSI) and is shown in Figure 24.22. IVF procedures produce a surplus of fertilized eggs and embryos that can be frozen and stored for future use. The procedures can also result in multiple births.
Human pregnancy begins with fertilization of an egg and proceeds through the three trimesters of gestation. The labor process has three stages (contractions, delivery of the fetus, expulsion of the placenta), each propelled by hormones. The first trimester lays down the basic structures of the body, including the limb buds, heart, eyes, and the liver. The second trimester continues the development of all of the organs and systems. The third trimester exhibits the greatest growth of the fetus and culminates in labor and delivery. Prevention of a pregnancy can be accomplished through a variety of methods including barriers, hormones, or other means. Assisted reproductive technologies may help individuals who have infertility problems.
- Nutrient and waste requirements for the developing fetus are handled during the first few weeks by:
- the placenta
- diffusion through the endometrium
- the chorion
- the blastocyst
- Progesterone is made during the third trimester by the:
- endometrial lining
- corpus luteum
- Which contraceptive method is 100 percent effective at preventing pregnancy?
- oral hormonal methods
- Which type of short term contraceptive method is generally more effective than others?
- natural family planning
- Which hormone is primarily responsible for the contractions during labor?
- Major organs begin to develop during which part of human gestation?
- first trimester
- second trimester
- third trimester
- Describe the major developments during each trimester of human gestation.
- Describe the stages of labor.
- The first trimester lays down the basic structures of the body, including the limb buds, heart, eyes, and the liver. The second trimester continues the development of all of the organs and systems established during the first trimester. The placenta takes over the production of estrogen and high levels of progesterone and handles the nutrient and waste requirements of the fetus. The third trimester exhibits the greatest growth of the fetus, culminating in labor and delivery.
- Stage one of labor results in the thinning of the cervix and the dilation of the cervical opening. Stage two delivers the baby, and stage three delivers the placenta.
contraception (also, birth control)
various means used to prevent pregnancy
length of time for fetal development to birth
human beta chorionic gonadotropin (β-HCG)
hormone produced by the chorion of the zygote that helps to maintain the corpus luteum and elevated levels of progesterone
inability to conceive, carry, and deliver children
organ that supports the diffusion of nutrients and waste between the mother’s and fetus’ blood
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