A fetus is the developing intrauterine form of a human or other mammal, loosely used to describe it from the development of the fertilized egg (embryo), but strictly referring in humans to the period from eight weeks gestation to birth. During fetal life, organ development is consolidated and specialization extended so that function may be sufficiently mature at birth; some organs start to function before birth in preparation for independent existence. During the fetal period most increase in size occurs, both in the fetus and in the placenta and uterus (or womb).
The fetus develops within the amniotic sac, which protects it and allows it to move about. Blood circulation in the fetus is adapted to the placenta as the source of oxygen and nutrients and site for waste excretion, but alternative channels are developed so that within moments of birth they can take over.