Abstract
Maternal plasma levels of progesterone and estradiol-17β were measured weekly by radioimmunoassay in 15 healthy women during the last trimester of normal pregnancy. There was a large individual variation for both hormones. The mean concentrations rose slightly with advancing gestational age but the ratio of progesterone to estradiol did not change. It is apparent that the onset of spontaneous labor cannot be predicted by measuring the levels of progesterone and estradiol-17β in maternal peripheral plasma.