Prenatal Development

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The end result of a successful pregnancy is that miracle called a child, which begins as a simple zygote and becomes a fertilized ovum during the first of three stages, or trimesters, of prenatal development. During the nine months, or approximately 266 days, of prenatal development, the zygote divides into billions of cells, which eventually become differentiated from one another while new systems and parts become integrated.

Ovum or Germinal Stage

Almost right after conception, cell division begins. While the zygote is splitting and new cells are created, it moves through the mother's fallopian tube toward the uterus, the place it will call home and where it will receive nourishment for the rest of its prenatal days. As with any other "egged" living thing, the yolk of the ovum provides all necessary nourishment.

By the time the cell cluster arrives at the uterus on the first stop of this journey, the process known as differentiation is just beginning. Here cells separate into groups according to their future roles. At this point the blastocyst (fertilized ovum) is a hollow ball of cells. Part of these cells will begin to form four membranes that help protect the growing organism. These membranes will eventually become the yolk sac, the allantois (which later becomes part of the circulatory system), the amnion (which soon forms the amniotic sac, the fetus's bubble-like home), and the chorion, which later becomes the placenta.

The blastocyst literally "floats" for some time in the uterus, and by the sixth day after conception it finds its home by implanting itself in the uterine wall. This is a critical point in gestation, because if the blastocyst does not implant itself properly and at the right time and in the right place, the cell mass will die before it can reach the embryo stage. If all goes well, the blastocyst will be firmly embedded about two weeks after conception.

Embryo Stage

During the forty-six-day embryo stage, the embryo grows to a length of more than one inch (2.5 centimeters). By the end of the embryo stage many body systems will be in operation, and the embryo will begin to appear human-like.

The embryo takes nourishment and oxygen and releases waste products through the umbilical cord, which links it with the placenta. The umbilical cord contains three blood vessels through which the embryo's blood circulates to and from the placenta.

The placenta is a disk-shaped mass of tissue six to eight inches (15.2 to 20.3 centimeters) long and one inch (2.5 centimeters) thick and weighs about one pound (.45 kilogram). Implanted in the inner wall of the uterus, it serves as a two-way filter between the bloodstream of the mother and the embryo. The placenta makes it possible for the mother to carry on life functions such as digestion, excretion, circulation, and respiration for the embryo. Into the placenta, by way of two arteries in the umbilical cord, the embryo deposits such waste material as carbon dioxide. The mass of blood vessels on the mother's side of the placenta then absorbs the wastes into her bloodstream. The embryo receives, through the vein in the umbilical cord, fresh nutrients (oxygen, amino acids, sugar, fats, and minerals) from the mother's bloodstream, and hormones, antibodies, and other necessary substances by the same route.

The placenta acts as a highly permeable membrane and as a natural screen to keep out many—but unfortunately, not all—harmful substances. Thus, the unborn child receives only materials with molecules that are small enough to pass through the screen. At the same time that the embryo is taking shape, the amniotic sac is developing into a protective chamber. By the end of the eighth week this sac completely surrounds the embryo. The watery fluid inside keeps the embryo from being jostled by any sudden movements of the mother or by accidents that may happen to her, such as a fall. The amniotic sac also keeps the embryo at a constant temperature.

During the embryonic period, three layers of cells are differentiated. The outer layer, or ectoderm, develops into sensory cells, skin, and the nervous system. The middle layer, or mesoderm, becomes the excretory system, muscles, and blood. The inner layer, the endoderm forms the digestive system, lungs, and thyroid gland.

By the end of the third week of development the embryo's heart is beating and its nervous system is forming rapidly. After the fourth week the legs are curled and the eyes have appeared as dark circles. During the fifth and sixth weeks arms and legs can be seen. After eight weeks all of the major body organs are present. The liver is making blood cells, and the kidneys are removing waste products. The mouth, nose, eyes, and head are clear and distinct. The head is roughly half the total body size at this time. Fingers and toes are blunt, and ribs show under the fetus' skin.

The eight-week time span of embryonic development is a particularly vulnerable period in human growth. Chemicals, drugs, hormones, or viruses present in the mother's system can very easily affect the embryo, as is described in more detail below in the section on "Prenatal Environmental Influences."

Fetal Stage

The fetal stage begins in the ninth week of pregnancy and continues until the birth of the baby, usually about thirty weeks later.

The thrill of the first-time mother feeling the movements of the fetus in the fourth or fifth month of pregnancy is unforgettable. By this time, the fetus can open and close its mouth, swallow, and make certain head movements. It may even suck its thumb. The fastest growth period for the fetus is the fourth month, when it almost doubles in length, reaching six inches (15.2 centimeters) from crown to rump. Limbs become sensitive to touch, and a heartbeat can be heard with a stethoscope, a thrilling and sometimes mystifying experience for parents-to-be.

The fetus is becoming more of an individual during this stage. Some fetuses move around a great deal while others are relatively quiet. The fetus' sense of hearing has also begun to function during this period, as evidenced by startled reactions to loud sounds.

After five months the skin of the fetus is fully developed. Hair, nails, and sweat glands are apparent, and the fetus even sleeps and wakes and keeps almost regular hours. In month six the eyelids can open, and the fetus can open and close its eyes. It may now weigh as much as twenty-four ounces (680 grams). During month seven the eyes can distinguish light from dark, and the brain has more control over body systems than before. The most important aspect of this particular time in development is that an infant born prematurely now has a fair chance for survival. In the last two months the fetus gains about eight ounces (227 grams) per week and gets ready for birth.

Prenatal Environmental Influences

With increasingly sophisticated technology, the fetus has been studied and is considered to be an active agent in its own development. Many scientists believe that anything that affects the environment of the fetus can have an effect upon development beginning at conception and not at birth.

Environment does indeed begin to influence the individual as soon as he or she is conceived. As the zygote undergoes mitosis (cell division), the new cells themselves become part of the mother's environment, and through their particular physical and chemical influence they guide and control the development of further new cells. Different genes are activated or suppressed in each cell, so that while one group of cells is developing into brain tissue, another is giving rise to the heart, another to the lungs, and another to the skeletal system. Meanwhile, the lump of cells is surrounded by the larger environment of the mother's uterus, and this environment is surrounded by the mother and the world in which she lives.

The great majority of women have uncomplicated pregnancies and give birth to healthy babies, and for many years it was believed that the baby in the uterus was completely insulated from outside influences. Scientists now know that this is not entirely true. Environmental influences ranging from radioactivity and stress in the outside world to drugs, chemicals, hormones, and viruses in the mother's bloodstream can affect prenatal development.

Such threats affect the health of the fetus in several ways. This is because the body organs and parts develop at different speeds and go through definite phases. First they go through a phase of rapid multiplication in the number of cells. Then there is an increase in both the number of cells and in cell size. In the third and final phase of development, cell size continues to increase rapidly, but cell division slows down.

When a body part or organ system is growing most rapidly both in cell number and size, this is known as a critical period. If an environmental factor, such as a chemical or virus, interferes with growth during the critical period, development of that organ system will be permanently affected.

The effect of environmental influences varies, therefore, in accordance with the stage of prenatal development in which the environmental factor is encountered, as well as the intensity of the threat, as shown in Table 1.

During the first three months of pregnancy, tissues and important body systems develop in the embryo. Adverse influences during this period will affect the basic structure and form of the body and may have particularly serious effects on the nervous system. Physical development can be arrested and irreparable malformation may occur. For example, many women who took the once-available drug Thalidomide during the first three months of pregnancy gave birth to children with serious defects. When taken toward the end of pregnancy, however, the drug did not seem to have any negative effects.


The scientific study of congenital abnormalities caused by prenatal environmental influences is known as teratology (from the Greek word teras, meaning "marvel" or "monster"), and the environmental agents that produce abnormalities in the developing fetus are called teratogens.


Chemicals (over-the-counter and prescribed pharmaceuticals as well as illegal substances) can cause a wide range of congenital abnormalities that account for about 10 percent of birth defects. The severity of the abnormality depends on the amount of the chemical the mother is exposed to, the developmental stage of the fetus, and the period of time over which the mother's exposure to the chemical takes place.

In terms of narcotics, women who are addicted to heroin, morphine, or methadone give birth to addicted babies. Soon after birth, the babies show symptoms of withdrawal, including tremors, convulsions, difficulty breathing, and intestinal disturbances.

Smoking and Nicotine

Cigarette smoking has already been shown to have dire consequences for the smoker, and it can be hazardous for the fetus and the newborn child. The results of studies of thousands of pregnancies in the United States and elsewhere, encompassing various ethnic, racial, and cultural groupings, indicate that the fetus and newborn are significantly affected by cigarette smoking during pregnancy.

Maternal smoking increases the risk of spontaneous abortions, bleeding during pregnancy, premature rupture of the amniotic sac, and fetal deaths and deaths of newborns. Women who smoke during pregnancy give birth to babies who are about one-half pound (225 grams) lighter (on the average) and smaller in all dimensions (for example, length and head circumference) than babies of nonsmokers, are born prematurely, and have other health problems.


The effects of alcohol are almost undisputed. Fetal alcohol syndrome (FAS), identified in 1973, is perhaps one of the best known and best documented outcomes of drinking, affecting approximately one out of every 750 births. And it is not just the heavy drinker who may place her fetus in danger. It has been found that women having one or more drinks daily were three times more likely to miscarry than women who had less than one drink daily.

FAS is a pattern of malformations in which the most serious effect is mental retardation. Other possible complications include permanent growth retardation, malformations of the face, brain damage, hyperactivity and learning disabilities, and heart defects.

Even if a child does not suffer from FAS, the effects of alcohol consumption can be significant. Although these children do not manifest the characteristics discussed above, they are at high risk for such problems of children of alcoholics as hyperactivity and learning disabilities. Results of research also indicate that moderate drinking can affect the later development of a child's intelligence as measured by IQ scores at age four.

The Mother

Since the mother's body is the chief element in the fetus' environment, the mother's physical condition can significantly affect the baby's development.

Among the maternal factors known to influence the fetus are disease, age, diet, reactions associated with a certain blood component, and prolonged stress.

Even a mother's knowledge of what is taking place in her body can be important. Some research has shown that mothers who consumed potentially teratogenic drugs during pregnancy had very little information about these drugs and even less information about their effect during pregnancy.


Since the placenta cannot filter out extremely small disease carriers, such as viruses, children can be born with malaria, measles, chicken pox, mumps, syphilis, or other venereal diseases that have been transmitted from the mother.

Rubella is the most widespread of the viruses that have a teratogenic effect. If a pregnant woman contracts rubella in the first three months of pregnancy, she is likely to give birth to a child with a congenital abnormality such as heart disease, cataracts, deafness, or mental retardation. Interestingly, there is not a direct relationship between the severity of the disease in the mother and its effect on the fetus. For example, women who have had mild attacks of rubella have given birth to babies with severe abnormalities.

Although rubella might be the most widespread disease, acquired immunodeficiency syndrome (AIDS) is by far the most frightening and the one that has received the most publicity. The vast majority of children with AIDS contracted the disease sometime between early pregnancy and birth. The disease is usually transmitted from the mother through the uterus during pregnancy or is acquired by the off-spring at birth.

As of the early twenty-first century, there was not a cure for AIDS, and the majority of efforts at controlling the disease focused on education in an effort to get potential female AIDS victims to take the proper precautions and avoid sexual relationships with high-risk males, usually those involved with drugs. Significant progress was being made, especially concerning children.

Toxemia is a frightening condition that is potentially fatal for the mother and the fetus. It is characterized by high blood pressure, swelling, and weight gain due to a buildup of fluid in the body tissues, and the presence of protein in the mother's urine. In severe cases the woman may go into convulsions or coma, placing a tremendous strain on her, which is carried over to the fetus. Women with toxemia frequently give birth to premature babies or to babies smaller than average for their gestational age. Like many other types of blood-pressure disorders, however, toxemia can be treated through medication and diet.

Anoxia is a condition in which the brain of the baby does not receive enough oxygen to allow it to develop properly. Anoxia can cause certain forms of epilepsy, mental deficiency, cerebral palsy, and behavior disorders. If the amount of brain damage is not too severe, however, it may be possible to compensate for the disorder to some extent. Epilepsy can often be controlled with drugs, for instance, and many children with cerebral palsy can learn to control their affected muscles.


Teenage mothers and those over thirty-five years of age have a higher risk of miscarriage, premature birth, and some birth defects than mothers in the prime childbearing years. Some of the reasons are fairly obvious. Very young mothers have not yet completed their own development, and the reproductive system may not be quite ready to function smoothly or effectively. In older women the reproductive system may be past its most efficient functioning.

In both cases, pregnancy puts an extra strain on a body that is not fully able to bear it. Furthermore, there is some reason to think that a woman's ova may deteriorate with age, leading to a greater risk of birth defects. Women have all their ova in partly developed form when they are born. So a woman who becomes pregnant at age thirty-seven, for example, is "using" an ovum that has been more or less exposed to thirty-seven years' worth of harmful chemicals, radiation, virus infections, and whatever else has happened to her body. This may explain why, for instance, Down syndrome is most common in children born to mothers over forty years of age.

It is quite possible that men's sperm may also be susceptible to chemicals and radiation effects over time. Furthermore, there may be genetic disorders that cause changes in sperm structure.

Diet and Physical Condition

Just as other aspects of physical health are important, so is the mother's diet. While physicians and researchers have long realized that pregnancy puts additional demands on the mother's body, they used to assume that the fetus' nutritional needs would be met first, even at the mother's expense.

The current opinion, however, is that the prenatal development of the fetus and its growth and development after birth are directly related to maternal diet. Women who follow nutritionally sound diets during pregnancy give birth to babies of normal or above-normal size. Their babies are less likely to contract bronchitis, pneumonia, or colds during early infancy and have better developed teeth and bones. The mothers have fewer complications during pregnancy and, on the average, spend less time in labor. The less time in labor, the easier the birth and the less stress the mother and child experience.

But if the mother's diet is low in certain vitamins and minerals when she is pregnant, the child may suffer from specific weaknesses. Insufficient iron may lead to anemia in the infant, and a low intake of calcium may cause poor bone formation. If there is an insufficient amount of protein in the mother's diet, the baby may be smaller than average and may suffer from mental retardation, with almost 20 percent fewer brain cells. Mothers who are also physically small (under 100 pounds [45 kilograms] in total body weight) are risky for pregnancy as well because of the stress that pregnancy can place on them.

The Rh Factor

The Rh-positive factor is an inherited genetically dominant trait in the blood that can result in a dangerous situation for the fetus. When blood containing the Rh factor (that is, Rh-positive blood) is introduced into blood without the Rh factor (Rh-negative blood), antibodies to combat the Rh factor are produced. If an Rh-negative woman mates with an Rh-positive man, the resulting child may have Rh-positive blood. Any small rupture in the capillaries of the placenta will release the Rh factor into the mother's bloodstream, causing her body to produce the antibodies needed to fight it. The antibodies in the mother's blood will then cross the placenta into the fetal bloodstream and attack its Rh-positive red blood cells, depriving the fetus of oxygen. The result may be a miscarriage, possible brain defects, or even death to the fetus or newborn child. Only in circumstances involving an Rh-negative mother and an Rh-positive child does this danger exist.

This condition in the child is called fetal erythroblastosis. Firstborn children are not threatened, because the mother's blood has not had time to produce a large amount of antibodies, but the risk increases with each pregnancy. In the past, erythroblastosis was always fatal, but now medical techniques can minimize the harmful effects of Rh incompatibility. After the birth of an Rh-positive child, the Rh-negative mother can be given an injection of the drug Rhogam to reduce the buildup of antibodies in her blood. If this is not done, future Rh-positive children will be endangered by the high antibody level. A doctor who suspects an Rh incompatibility between mother and fetus can measure her antibody level and induce labor if the antibody count becomes too high. Immediate and complete blood transfusions to the newborn infant can then eliminate the mother's antibodies from its blood. If the fetus is not yet mature enough to survive after birth, a blood transfusion may be possible in utero.


If a mother is extremely anxious (about her pregnancy, her abilities as a mother, or any other problems in her life), the unborn child may be affected. Although the baby's nervous system is separate from the mother's, strong emotions in the mother such as rage, fear, and anxiety cause a great increase of hormones and other chemicals in her bloodstream. These substances pass through the placenta wall, and it is believed that they can reproduce the mother's physiological state in the fetus.


The benefits of exercise are unquestioned. People who regularly exercise live longer for a variety of reasons. Their circulatory and respiratory systems get exercise and function more efficiently. Exercise tends to increase energy, make sex more enjoyable, decrease depression, increase self-confidence, and suppress appetite, which helps maintain an ideal weight-to-height ratio. There is also a social component to exercise (getting out of the house, being with other adults) that provides an incentive to continue. When the human body is in better condition, it can better fight off diseases, and when struck with illness, it reacts in a more positive fashion.

One longitudinal study examined the relationship between exercise and pregnancy, focusing on the levels of fatigue that women experience during childbirth and the association between fatigue and several maternal factors. The best predictors for fatigue among these women were examined. The set of possible predictors included whether the women took formal childbirth education classes, their average amount of sleep, whether they exercised during the last trimester of pregnancy, whether they worked during the last trimester of pregnancy, their anxiety level, and whether they had been medicated prior to the interview. When admitted to the hospital, the best predictors of fatigue were anxiety, amount of sleep, and medication. Two hours after admission, the best predictors were previous fatigue, medication, and work. Four hours after admission, the variables were medication, previous fatigue, exercise, and childbirth education. Six hours after admission, the predictors with the greatest influence were previous fatigue, medication, exercise, and work. Within twenty-four hours after delivery, the greatest predictor of fatigue was previous fatigue.

Although exercise in a couple of instances seemed to be associated with fatigue, for the most part it did not seem to be one of the best predictors for fatigue. Along with the other benefits mentioned earlier, it would seem that exercise helps lessen a woman's fatigue during childbirth.


Never before has the importance of prenatal care for the developing child been so apparent. With increases in research and knowledge of what factors can affect the zygote, embryo, and fetus, the likelihood that such factors can have a negative impact on the well-being of the child and the mother is greatly decreased.



Boyd, Susan. Mothers and Illicit Drugs: Transcending the Myths. Toronto: University of Toronto Press, 1999.

"Pregnancy and Prenatal Care." In the Make Way for Baby [web site]. Pompano Beach, Florida, 2000. Available from; INTERNET.

"Prenatal Development and Birth." In the George Mason University [web site]. Fairfax, Virginia, 2001. Available from; INTERNET.

Priest, Judy, and Kathy Attawell. Drugs in Conception, Pregnancy, and Childbirth. London: Thorsons Publications, 1998.

Neil J.Salkind