The effect of human chorionic gonadotropin and pregnancy on the circulating level of relaxin.

The effect of hCG and pregnancy on the circulating levels of relaxin was investigated in 48 women undergoing in vitro fertilization and embryo transfer (ET) for the treatment of infertility. Subjects were allocated randomly to receive hCG (Profasi; 2000 IU, im) or placebo on the day of ET (day 0) and on day 3 after ET (day 3). Samples of peripheral blood were taken on days -4, 6, and 10. An additional sample was taken on day 22 from women who became pregnant. The subjects were allocated retrospectively to 1 of 4 groups: no pregnancy, no hCG (NP); pregnancy, no hCG (P); no pregnancy, hCG (NPH); and pregnancy, hCG (PH). Two patients with blighted ova and one with a tubal pregnancy were excluded from the analysis. The concentrations of relaxin were similar and rose significantly in all groups at each time point (P less than 0.05). On day 6 there was no significant difference in the increment between the groups, but by day 10, circulating levels in the P compared to the NP and those in the PH compared to the NPH group were significantly greater (P less than 0.05). By day 22 the difference between the PH and the P groups was significant (P less than 0.05). Relaxin levels correlated with progesterone levels on day 10 in the NP and P groups (r = 0.633; P less than 0.05 and r = 0.697; P less than 0.05, respectively) and with estradiol levels in the P group only on days 6 and 22 (r = 0.659; P less than 0.05 and r = 0.783; P less than 0.05, respectively). These data demonstrate that in women undergoing in vitro fertilization, relaxin levels increase during the luteal phase, and in those women who establish a pregnancy, the values are significantly greater by day 10. The administration of hCG in the early luteal phase, before implantation, to women who subsequently become pregnant significantly increased the level of circulating relaxin on day 22. The positive correlation between relaxin levels and ovarian steroid levels in the groups not receiving exogenous hCG suggests that a common factor may control the release of both.