The unborn child from the start of the ninth week after conception to the time of birth.

Prenatal development is defined by two stages. The first stage involves the formation of the embryo from the fertilized egg. During this stage, the embryo's cells multiply and differentiate rapidly at an exponential rate. The embryo develops a primitive nervous and circulatory system. In the second stage of prenatal development the child-to-be is referred to as the fetus. Embryologists begin calling the embryo a fetus once the first bone cells appear, which is at about the eighth week after conception. The fetus is unlike the embryo in that it begins to become much more active. In the first trimester the fetus can kick its legs and move its arms. At this stage the heartbeat is strong enough to be detected by ultrasound.

For Further Study

Books

Johnson, Robert., ed. Mayo Clinic Complete Book of Pregnancy and Baby's First Year. New York: William Morrow, 1994.

Simkin, P., J. Whalley, and A. Keppler. Pregnancy, Childbirth, and the Newborn: The Complete Guide. New York: Meadowbrook Press, 1991.