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

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.

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.

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.

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.

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.

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.