Concentrations of Estradiol (E2), progesterone (P), 17-hydroxyprogesterone (17-OHP), follicle-stimulating hormone (FSH), luteinizing hormone-human chorionic gonadotropin (LH-HCG), and human placental lactogen (HPL) were measured by radioimunoassay in serum samples obtained daily from 3 women from the last menses prior to conception throughout the first few months of gestation. FSH levels declined after implantation in one subject and remained unchanged in the others. HCG levels began to rise above luteal phase LH values 11 to 14 days after the mid-cycle LH peak. HPL became detectable 34 to 38 days after the LH peak. E2 levels rose steadily after the postovulatory nadir, increasing more rapidly 4 weeks after the LH peak. The patterns of P and 17-OHP were initially similar. Both rose after ovulation, remained elevated for several weeks, and then declined. About 11 weeks after the last menstrual period, levels of P again increased while 17-OHP remained low. Single serum samples were obtained from an additional 159 gravid women 6 to 16 weeks after the last menstrual period, and the 3 steroids were measured in each. Mean levels of E2 steadily increased and 17-OHP steadily decreased. Mean P dropped to a nadir at 9 weeks and rose thereafter. Since 17-OHP is mainly of luteal origin, these findings indicate that in normal pregnancy the corpus luteum has maximal activity for about 4 weeks after ovulation. Falling levels of P in the next 3 weeks prior to increased trophoblast production of this hormone may contribute to the cause of first-trimester uterine bleeding in some women who have normal term pregnancies.
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