Oestrogen and progesterone receptor concentrations in human endometrium during gestation.

The concentrations of endometrial oestrogen and progesterone receptors, both in cytosol and in nuclei, have been studied at 8--10 weeks and at 38--40 weeks of gestation. At these two periods the concentration of oestrogen receptors is comparable with the concentration observed during the late secretory phase of the menstrual cycle. At 8--10 weeks of gestation, concentration of progesterone receptors is also comparable with the concentration observed during the secretory phase of the menstrual cycle, but at term there is a significant increase (P less than 0.05) and the concentration is then comparable with the concentration observed in the pre-ovulatory period of the menstrual cycle. The receptor binding sites are always predominantly found in nuclei and the increase in progesterone nuclear receptor at term suggests that in man the progesterone withdrawal is not a necessary step in the mechanism of uterine activation during parturition as it is in other species.