Both theory and animal experiments often relate impairment of fertility and fetal development to the stress of hypoxia. This comprehensive, penetrating study includes critical reviews of theory and experiments as well as of previous studies of human birth weight and neonatal mortality at high altitudes in the United States. It also reports on studies of mothers, placentas, and the newborn in two Peruvian populations, one in the Andes, the other at sea level. It presents new evidence on the functional significance of various aspects of placental anatomy and on racial differences in maternal ability to nourish the fetus at high altitudes.