What is prenatal physical development?

Introduction

Pregnancy encompasses the development of a single-celled fertilized egg into a trillion-celled baby. The many changes that transform the fertilized egg into a newborn infant over nine months of human pregnancy constitute prenatal development. Prenatal development comprises three stages (zygote, embryo, and fetus) and is also commonly categorized into three trimesters, each lasting three months. Although prenatal development typically follows a predictable course, development can be disrupted by both genetic and environmental factors. This disruption may result in a range of outcomes, from fetal death and severe abnormalities such as deformed or missing limbs, to minor abnormalities such as low birth weight and neurological dysfunction such as learning disabilities or attention-deficit hyperactivity disorder (ADHD).

The fourth century b.c.e. Greek philosopher Aristotle is regarded as the first in Western civilization to study prenatal development. In the years that followed, others superficially investigated the topic. However, it was not until the beginning of the twentieth century that researchers intensified their study of prenatal development. In the early 1900s, researchers were significantly influenced by the evolutionary theories of Charles Darwin and believed that all aspects of prenatal development were genetically determined.

Josef Warkany, a pioneering American scientist, engendered a shift in the thinking of the scientific community during the 1940s. Warkany documented that environmental factors, called teratogens, could adversely affect prenatal development and cause malformations at birth. About a decade later, the notion that environmental factors could harm prenatal development became mainstream after the 1950s thalidomide tragedy. Thalidomide was a drug given to pregnant women to combat symptoms of nausea. When taken in the first trimester of pregnancy, the drug produced severe physical deformities in infants, including missing arms and stunted limbs, and its use was subsequently banned. Following this tragedy and the resulting acceleration in understanding of the importance of intrauterine life, diagnostic tests have become routinely used to monitor the course of prenatal development.

Stages of Prenatal Development

Prenatal development begins when a sperm successfully fertilizes an egg (ovum) and usually lasts an average of thirty-eight weeks (nine months). The American College of Obstetrics and Gynecology has standardized the terminology used to describe the three stages of prenatal development. The first stage, the zygote (or germinal) stage, begins at fertilization and ends two weeks later, shortly after implantation of the zygote in the uterine wall. The second stage, the embryo stage (weeks three to eight), is the most vulnerable to teratogenic (environmental) insult. The fetal stage (weeks nine to thirty-eight) represents the final and longest stage of prenatal development.

The Zygote Stage

Fertilization of an egg by a sperm creates a zygote. The two-week period of the zygote after conception ends with its implantation into the uterine wall. During these two weeks, the zygote grows rapidly and is carried by currents in one of the Fallopian tubes toward the uterus. The movement through the Fallopian tube usually takes five days. The zygote divides from a single cell into a mass of approximately one hundred cells. Approximately one week after fertilization, the zygote is ready to attach itself to the uterine wall. Many potential pregnancies terminate at this point as a result of implantation failure. Implantation takes approximately one week to complete, connects the zygote with the woman’s blood supply, and triggers hormonal changes that prevent menstruation. At this stage, the implanted zygote is less than a millimeter in diameter but is beginning to differentiate into two structures: the germinal disc and the placenta. The germinal disc eventually develops into the baby, while the remaining cells transform into the placenta. The placenta is the structure through which nutrients and waste are exchanged between the mother and the developing child. Successful implantation and differentiation into the placenta and germinal disc mark the end of the period of the zygote.

The Embryo Stage

On successful implantation in the uterine wall, the zygote is called an embryo and pregnancy enters its second stage. The embryo stage typically begins three weeks after conception (fertilization) and lasts through the eighth week of pregnancy. At the beginning of the embryo stage, the embryo is only two millimeters long and less than an ounce in weight. The embryo is enclosed in a protective sac called the amnion, which is filled with amniotic fluid that cushions and maintains a constant temperature for the embryo. The embryo’s cells form into three layers: The outer layer (ectoderm) becomes the hair, the outer layer of skin, and the nervous system; the middle layer (mesoderm) forms muscles, bones, and the circulatory system; and the inner layer (endoderm) forms the digestive system and lungs. At the beginning of this stage, the embryo looks more like a lizard than a human being, as a result of the shape of its body and head. By the end of the eighth week of pregnancy, the embryo manifests distinguishable human characteristics (eyes, arms, legs) and contains in rudimentary form all of its organs and body structures. Despite these significant changes, the embryo remains too small to be detected by the mother.

The Fetal Period

The longest and final phase of prenatal development is known as the fetal period. The fetal period represents a time when the finishing touches are put on the structures of the fetus. This period begins at nine weeks and ends with the birth of the baby. During this stage of pregnancy, the growth and development of the fetus is astounding. The fetus will increase in mass from less than one ounce at week nine, to eight ounces at four months, and to nearly eight pounds at birth. Around the start of the fetal period, the fetus begins to differentiate sex characteristics. At twelve weeks, the circulatory system becomes functional. At sixteen weeks, the mother can detect fetal movements known as quickening. By twenty weeks, a fine layer of hair (called lanugo) begins to grow over most of the fetus’s body. Sucking and swallowing reflexes are present by twenty-four weeks of gestation. Brain specialization becomes particularly acute by about twenty-eight weeks. At thirty-two weeks of gestation (seven months), the fetus is viable outside the mother’s womb. By this time, most systems function well enough that a fetus born at this age has a chance to survive. Despite the potential to survive, premature birth predisposes a baby to myriad additional developmental problems (health problems, learning disabilities, and cognitive deficits). By thirty-two weeks of prenatal development, the fetus has regular periods of physical activity, and the eyes and ears begin to function. By thirty-six weeks of gestation, the fetus experiences rapid weight gain, and development consists largely of an increase in weight and length. At approximately thirty-eight weeks of gestation, birth will occur. The average newborn baby weighs between seven and eight pounds.

Disruptions in Prenatal Development

Although most of prenatal development progresses in a healthy and predictable fashion, numerous factors can disrupt the course of prenatal development. It is customary to divide the possible cause of these malformations into genetic factors (chromosomal abnormalities) and environmental factors (such as drugs or viral infections). There is often an interaction between environmental conditions and genetic factors such that the environment can either exacerbate or mitigate any potential adverse outcomes. The impact of both genetic and environmental factors may result in abnormalities that range from fetal death and severe structural defects to subtle neurological malformations that may not manifest themselves until several years after birth (as with learning disabilities or ADHD).

General Risk Factors

Parental age can have an impact on prenatal development. Women over the age of thirty-five are at greater risk of giving birth to children with birth defects such as Down syndrome and other chromosomal abnormalities. Recent research suggests that older men also have an increased risk of fathering children with birth defects as a result of the presence of damaged sperm that may fertilize the egg. Teenage girls are also at greater risk for giving birth to children with birth defects as a result of poor maternal health and inadequate prenatal care. When prenatal nourishment and care are lacking, the baby is more likely to be born prematurely, have a lower birth weight, and be at greater risk for learning difficulties and a host of behavioral and emotional problems. Recent research has also implicated other factors during pregnancy as a general risk factor for psychological, behavioral, and educational outcomes.

Genetic and Chromosomal Risk Factors

Thousands of genetic and chromosomal anomalies can potentially disturb normal prenatal development. Although many of the causes of genetic and chromosomal abnormalities are unknown, some may be attributable to exposure to teratogens that damage the chromosomes during prenatal development. Research emerging out of the Human Genome Project is continuously documenting additional chromosomal abnormalities that may have an impact on prenatal development. A chromosome is a microscopic component of a cell that carries its genetic makeup. One of the most common chromosomal disorders is Down syndrome. Individuals with Down syndrome have slanted eyes; thick, fissured tongues; and a flat, broad face. They are often mentally disabled and have significant language impairments. Other chromosomal and genetic disorders include Turner syndrome, Klinefelter syndrome, fragile X syndrome, muscular dystrophy, and neural tube defects that result in spina bifida. Many of these conditions produce intellectual disability and physical anomalies such as brain damage, unusual appearance, and malformed limbs.

Prenatal Diagnostic Tests

Prenatal diagnosis of potential problems is possible using tests, such as amniocentesis, that can detect the presence of many chromosomal and genetic abnormalities. Amniocentesis involves the insertion of a hollow needle through the mother’s abdomen into the amniotic sac and the withdrawal of fluid containing fetal cells. Amniocentesis can detect chromosomal abnormalities such as Down syndrome, but it is not usually performed until the fifteenth week of pregnancy. Chorionic villus sampling (CVS) provides the same information as amniocentesis, but at a much earlier gestational period (seven weeks). In CVS, fetal cells are obtained from the placenta by means of a tube inserted through the vagina. There is greater risk of infection and miscarriage with CVS. Fetoscopy is a surgical procedure involving the insertion of an instrument that permits actual viewing of the fetus and the obtaining of fetal tissue. This procedure is more precise than CVS and amniocentesis but carries a highter risk of miscarriage.

Ultrasound involves the use of sound waves that provide a computer-enhanced image of the fetus. It is a noninvasive, painless, and low-risk procedure that provides an actual image of fetal shape and movement. It is useful for detecting normal and abnormal fetal development and for determining fetal position and age. Preimplantation diagnosis is an experimental, highly technical genetic examination of cells before their implantation in the uterine wall. It typically follows in vitro fertilization and permits the detection of specific genetic disorders. In the future, it may be useful for correcting genetic disorders as well.

Through prenatal diagnostic tests, researchers are able to detect genetic weaknesses (and strengths) from the earliest moments of life. Researchers have also begun to experiment with ways of altering genetic messages, the results of which may lead to corrections of genetic abnormalities in the future.

Environmental Risk Factors

A teratogen is an environmental agent such as alcohol, cocaine, or infectious organism that has an adverse impact on prenatal development following maternal exposure. The word has Greek origins and literally means “monster-forming.” Certain stages of prenatal development are more vulnerable to teratogens than others. Exposure during the period of the zygote usually results in spontaneous abortion of the fertilized egg, while exposure during the embryo stage can lead to major defects in bodily structure and quite possibly death. Exposure during the fetal period usually produces minor structural defects, such as wide-set eyes, and neurological impairment, such as intellectual disability or learning problems. Some of the more commonly implicated teratogens include infectious agents such as cytomegalovirus, varicella virus, and human parvovirus B19, and drugs such as alcohol, cocaine, and nicotine.

The fetus is most vulnerable to the effects of teratogens during the first trimester. These effects are severe and may result in structural deformities and death.

Sarnoff Mednick and others at the University of Southern California reported on a more subtle form of prenatal disturbance following second trimester exposure. Mednick reported preliminary data that linked second trimester viral infections to later psychological outcomes such as depression and schizophrenia. Jose Cordero, former U.S. assistant surgeon general and director of the CDC’s Center for Birth Defects and Disabilities, indicated greater need for awareness of a broader range of teratogens such as fever and infectious agents in relation to outcomes such as learning disabilities, mood disorders, and attention deficits. Stefan Dombrowski, a professor at Rider University in New Jersey, and Roy Martin, a professor at the University of Georgia, compiled the first book on the topic of prenatal exposures in relation to psychological, behavioral, and educational outcomes in children. These two researchers indicate that certain prenatal exposures including fever, influenza, stress, and air pollution may be associated with adverse psychological, behavioral, and educational outcomes. The hypothesis guiding this research is that a prenatal exposure disrupts the neurological development of the fetus and produces abnormal behavioral and psychological outcomes in offspring.

There are additional environmental agents that can potentially disrupt the normal course of prenatal development. Studies have investigated the impact of caffeine. Although the results are equivocal, exposure to moderate amounts of caffeine may result in lower birth weight and decreased fetal muscle tone. Excessive caffeine use during pregnancy should, therefore, be avoided. The impact of alcohol during pregnancy is well documented. Chronic alcohol use produces fetal alcohol syndrome and associated cognitive deficits and physical deficits such as heart problems, retarded growth, and misshapen faces. Maternal alcohol use during prenatal development is the most common cause of intellectual disability. Because even moderate daily alcohol use (two ounces of alcohol) has been associated with some of these outcomes, it is recommended that alcohol use during pregnancy be avoided. Nicotine exposure from cigarette smoking is another well-established teratogen. Research indicates that prenatal cigarette exposure increases the risk for low birth weight, cognitive deficits, learning problems, behavior problems, and even fetal death as a result of nicotine-induced placental and neurological defects.

Overall, the critical prenatal period for exposure to teratogens is during the first trimester of pregnancy. Within the first trimester, certain periods are even more sensitive to teratogens than others. For example, the first six weeks of pregnancy is a particularly sensitive period in the development of the central nervous system, while the eyes are vulnerable during weeks five through eight. It is commonly accepted that exposure to teratogens during the first eight weeks of pregnancy may induce major structural abnormalities. Exposure during the remainder of prenatal development, depending on the type of teratogen and intensity and duration of exposure, may lead to minor structural abnormalities (wide eyes, webbed hands) as well as cognitive, behavioral, and psychological difficulties. Although central nervous system development and brain growth are most vulnerable to disruptions during the first trimester of pregnancy, these structures continue to develop throughout the prenatal period. Thus, exposure to any environmental risk factor should be minimized if at all possible.

Bibliography

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