Prenatal development: The process of growth and development within the womb, in which a single-cell zygote (the cell formed by the combination of a sperm and an egg) becomes an embryo, a fetus, and then a baby. The first two weeks of development are concerned with simple cell multiplication. This tiny mass of cells then adheres to the inside wall of the uterus. The next three weeks see intense cell differentiation, as the cell mass divides into separate primitive systems. At the end of eight weeks, the embryo has taken on a roughly human shape, and is called a fetus. For the next twenty weeks the fetus' primitive circulatory, nervous, pulmonary, and other systems become more mature, and it begins to move its limbs. At 28 weeks, fat begins to accumulate under the skin, toenails and fingernails appear, and downy hair sprouts on the body and scalp. The fetus may open its eyes periodically. For the remaining weeks of development, the fetus continues to gain weight, and its internal systems reach full development.