The relationship between hCG and relaxin secretion in normal pregnancies vs peri-implantation spontaneous abortions

Objective: We determined the ovarian response to human chorionic gonadotrophin (hCG) in terms of relaxin and progesterone secretion during the peri-implantation period of normal and failing pregnancies. We wished to test the hypotheses that relaxin production in failing pregnancies is different from that in normal pregnancies, that relaxin is a reliable, quantitative indicator of the biological activity of endogenous hCG, and that relaxin is a useful predictor of peri-implantation spontaneous abortions. Design: Daily blood samples were collected in a prospective longitudinal study from insemination patients. Patients: Women undergoing artificial insemination in natural cycles with non-frozen donor semen at a University clinic. Measurements: Serum LH, hCG, relaxin and progesterone were measured and the relationship between hCG and the ovarian hormones was evaluated in the peri-implantation period of normal pregnancies and spontaneous abortions. Results: Nine of 23 conceptive cycles resulted in a spontaneous abortion between 16 and 70 days after the LH peak. In all normal and failing pregnancies there was a close qualitative relationship between hCG secretion and relaxin production. Six of nine failing pregnancies were associated with abnormally low hCG secretion. Six of the spontaneous abortions were associated with rates of relaxin secretion which were higher than the mean of 14 normal pregnancies. No such alterations in progesterone concentrations were observed. In cases where hCG was extremely low, the quantitative relationship between hCG and relaxin was different from that in cases of normal hCG concentrations. Conclusions: There is a close temporal relationship between the secretion of trophoblastic hCG and ovarian secretion of relaxin in the peri-implantation period of normal and failing pregnancies. In failing pregnancies there is substantial variability in the quantitative relationship between relaxin and hCG, indicating that relaxin is not a reliable quantitative indicator of hCG bioactivity. Contrary to previous reports, relaxin concentrations in failing pregnancies tended to be higher than or equal to concentrations in normal pregnancies until the loss was imminent. Because of this relaxin is not a useful predictor of peri-implantation spontaneous abortions.