Cholesterol is required for fetal development. Data obtained from recent studies in humans, rodents, and cell cultures showed that circulating maternal cholesterol can affect fetal metabolism and sterol accretion. Recent studies in our laboratory showed that the efflux of cholesterol from the basolateral side of the placental cells and the secretion of cholesterol from endodermal yolk sac cells to the fetal circulation can be regulated. The ability to manipulate the mass of maternal cholesterol that crosses to the fetus could result in a dramatic improvement in the development of fetuses that lack the ability to synthesize cholesterol, such as those with Smith-Lemli-Opitz syndrome. On the other hand, it could also accelerate the development of various age-related diseases, such as atherosclerosis.