Apparent Secretion of Progesterone by Human and Goat Fetuses1

IF, AS is usually conceded, the placenta secretes progesterone, one may ask how much of the hormone reaches the fetus. The following experiments were undertaken to study this problem; the data which resulted partially answered the question, but also disclosed an unexpected and interesting fetal-placental relationship. A preliminary report has appeared (Forbes, 1954a). Ober, Klein, and Weber have recently (1954) presented evidence that the sensitivity of the progesterone assay described by Hooker and Forbes (1947) may be increased up to five-fold in the presence of progesterone: estradiol mixtures in the ratio of 500:1. Little is known regarding circulating levels of endogenous estrogens; it is possible that they may occur in concentrations which would disturb the accuracy of the bio-assay. The writer proposes, therefore, when using this assay to express his results in terms of progesterone equivalents (PE). One progesterone equivalent is defined as equal to the activity of 1 μg.