The human fetus is exposed to increasing levels of steroids as pregnancy progresses. Conjugation is probably the major mode of inactivation. Each important portal of entry into the fetus is an active tissue or organ of conjugation; the liver receives steroids from the umbilical circulation, and the lungs, gastro-intestinal tract and skin from the amniotic fluid. The ability of the fetus to metabolize conjugates directly without prior hydrolysis is another possible mechanism of inactivation. Conjugates are poorly transferred to the mother. However, the placenta and fetal membranes are endowed with active sulfatases which hydrolyze sulfates, thus facilitating transfer to the mother. Glucosiduronates, synthesized to a limited extent by the fetus, are poorly transferred and not hydrolyzed by the membranes and placenta, perhaps accounting for their high concentration in amniotic fluid.