Human biological foundations include the genetic code and basic genetic principles that contribute to individual differences in appearance and behavior and to various abnormalities and disorders. During the most rapid phase of human growth, the prenatal period, complex transactions between heredity and environment begin to shape the course of development.

Genetic counseling and prenatal diagnosis help people make informed decisions about conceiving or carrying a pregnancy to term. The vast changes that take place during pregnancy are divided into three periods: the period of the zygote, the period of the embryo, and the period of the fetus. Environmental supports such as maternal health and nutrition are necessary for normal prenatal growth, but damaging influences such as teratogens, inadequate maternal diet, and severe maternal emotional stress can threaten the child's health and survival. Prenatal vitamin supplements can benefit women during pregnancy and also prevent certain birth complications and defects.

Childbirth takes place in three stages, beginning with contractions that open the cervix and ending with the delivery of the placenta. Natural, or prepared, childbirth tries to overcome the idea that birth is a painful ordeal that requires extensive medical intervention. Interest in home birth, which has always been popular in certain industrialized nations, has increased in the United States. In complicated deliveries, labor and delivery medication is essential; but used routinely, it can cause problems. Oxygen deprivation, prematurity, and low birth weight are serious birth complications. Infants born underweight or prematurely face developmental risks, but interventions to ensure a supportive home environment can help restore these children's growth.

The field of behavioral genetics is devoted to discovering the hereditary and environmental origins of the great diversity of complex human characteristics. These two basic determinants of development continue to influence the individual's emerging characteristics from infancy through adolescence.