Fetus, unborn or unhatched vertebrate whose basis structural plan is in place; in humans, the period from 3 months' gestation to birth. The fetus lives in a sac of amniotic fluid that protects it and allows it to move about. Blood circulation takes place via the placenta and the umbilical cord—the sources of oxygen and nutrients and the means for waste excretion. During fetal life, organ development is consolidated so that function may be sufficiently mature at birth.
See also: Reproduction.