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Both genetic and extrinsic factors influence normal and abnormal embryonic and fetal growth

This is an excerpt from Life Span Motor Development 6th Edition With Web Study Guide by Kathleen Haywood & Nancy Getchell.

Prenatal Development

The growth process begins the instant an ovum (egg) and spermatozoon fuse in fertilization. Carried out under the control of genes, early development is astonishingly precise. Genes, then, determine the normal aspects of development and inherited abnormal development. At the same time, the growing embryo (and, later, the fetus) is very sensitive to extrinsic factors, which include the environment in which the fetus is growing - the amniotic sac in the uterus - and the nutrients delivered to the fetus via the mother's circulation and the placenta. Even in the womb, individual genetic factors and extrinsic factors interact in the fetus' development. Some extrinsic factors, such as abnormal external pressure applied to the mother's abdomen or the presence of certain viruses and drugs in the mother's bloodstream, are detrimental to the fetus. Other factors, such as delivery of all the proper nutrients, enhance the fetus' growth.

Prenatal growth is divided into two phases: embryonic growth, from conception to 8 weeks, and fetal growth, from 8 weeks to birth. Let's consider the key features of each phase.

Embryonic Development

Development begins with the fusion of two sex cells: an ovum from the female and a spermatozoon from the male (see figure 4.1). The genes direct the continuous development of the embryo in a precise and predictable pattern.
As the embryo moves through the oviduct, its cells divide and multiply. By the time it implants on the lining of the uterus, it is several hundred cells in size. It is embedded in nutrient cells that nourish it. Implantation in the uterus is facilitated by protuberances of sugar molecules on the surface of the blastocyst.

The number of cells increases, and the cells differentiate to form specific tissues and organs. This process occurs in a predictable time line, summarized in table 4.1. At 4 weeks, the limbs are roughly formed and the heartbeat begins. By approximately 8 weeks, the eyes, ears, nose, mouth, fingers, and toes are formed. By this time, the human form has taken shape.

Fetal Development

The fetal stage, from 8 weeks to birth, is characterized by further growth and cell differentiation of the fetus, leading to functional capacity. This continued growth of the organs and tissues occurs in two ways: by hyperplasia and by hypertrophy. If you examine the landmarks of growth carefully, you will also see that growth tends to proceed in two directions. One direction is cephalocaudal, meaning that the head and facial structures grow fastest, followed by the upper body and then by the relatively slow-growing lower body. At the same time, growth is proximodistal in direction, meaning that the trunk tends to advance, then the nearest parts of the limbs, and finally the distal parts of the limbs (figure 4.2). Body weight increases and the body tissues grow steadily, with the rate of growth increasing at about 5 months and continuing at that rapid rate until birth.
A fetus at 3 months.

Differentiation is the process wherein cells become specialized, forming specific tissues and organs.

Hyperplasia is an increase in the absolute number of cells.

Hypertrophy is an increase in the relative size of an individual cell.

Cephalocaudal is the direction of growth beginning at the head and extending toward the lower body.

Proximodistal is the direction of growth proceeding from the body toward the extremities.

Although cells differentiate during growth to perform a specialized function, some cells have an amazing quality termed plasticity, which is the capability to take on a new function. If some of the cells in a system are injured, for example, the remaining cells might be stimulated to perform the role that the damaged cells would ordinarily carry out. The cells of the central nervous system have a high degree of plasticity, and their structure, chemistry, and function can be modified both prenatally and postnatally (Ratey, 2001).

Plasticity is modifiability or malleability; in regard to growth, it is the ability of tissues to subsume functions otherwise carried out by other tissues.

Fetal Nourishment

Many characteristics of the fetal environment have the potential to affect growth, either positively or negatively, and the nourishment system is the extrinsic factor that has the most influence on fetal development. The fetus is nourished by the diffusion of oxygen and nutrients between fetal blood and maternal blood in the placenta (figure 4.3). Carbon dioxide and excretory byproducts also are exchanged and carried away in the mother's blood.
A diagram of the placenta showing the two circulations - that of mother and that of fetus - which come close enough to each other that substances diffuse from one to the other, though the bloodstreams never mingle.

Reprinted by permission from Rhodes 1969.

The growing fetus needs energy, nutrients, and oxygen. If these are in short supply, mother and fetus compete for limited resources, possibly compromising the needs of the fetus. Obviously, then, maternal health status plays a role in prenatal development.

A woman who lives in better conditions (with an adequate, safe food supply and a protective, clean environment) and who receives early prenatal health care is more likely than a woman living in poorer conditions to meet the needs of the fetus. She is also more likely to be at lower risk for illnesses and infections that might compromise the health of the fetus and would result in low birth weight. Consequently, women at lower socioeconomic levels typically give birth to lighter infants than do women at higher socioeconomic levels. This is significant because low-birth-weight infants are at greater risk of disease, infection, and death in the weeks after birth than are normal-weight infants. Some of the differences in birth weight among ethnic groups can be attributed to parental height and are thus largely influenced by genetic factors (Troe et al., 2007). Further research is needed to distinguish the influences on birth weight that are primarily genetic from those that are environmental and thus might be modifiable to promote health in the early postnatal period.

If you were a doctor, would you have to travel to a poor country with primitive living conditions to treat pregnant women with poor health status? Or would you likely see some poor women in your practice in an affluent country? What groups of pregnant women in an affluent country might be at risk of poor health?

Read more from Life Span Motor Development, Sixth Edition by Kathleen M. Haywood and Nancy Getchell.

More Excerpts From Life Span Motor Development 6th Edition With Web Study Guide