Of all the endocrine glands in the human fetus, the adrenal has perhaps aroused the greatest interest. The human fetal adrenal (HFA) glands secrete very large quantities of steroid hormones; indeed it can be computed that the adrenals of some human fetuses near term secrete 100–200 mg steroids daily, most of which is dehydroisoandrosterone sulfate (DS). The rate of steroid secretion by the fetal adrenals may be 5 times that of the adrenals of adults at rest (1, 2). During human pregnancy the rate of estrogen production increases strikingly. In fact, the production rate of estriol in near-term pregnant women is 1000-fold greater than that in nonpregnant women (3). The site of estrogen production is the placenta, and the precursor of these large quantities of estrogen has been shown to be DS that is principally of fetal origin (4). Ryan (5) demonstrated that the aromatase activity of placental tissue was high and Cig-steroids were converted to estrogens rapidly in trophoblasts.