Relaxin secretion in in vitro fertilization pregnancies

OBJECTIVE: This study was designed to determine whether the late luteal functional status of the corpora lutea in in vitro fertilization cycles alters the secretion of relaxin during pregnancy. STUDY DESIGN: Analysis of serum relaxin, human chorionic gonadotropin, and steroid concentrations in sera of women with pregnancies viable beyond the twelfth week as a result of in vitro fertilization treatment was performed. RESULTS: The serum estradiol and progesterone concentrations decreased 5.5- and 4-fold from days 5 to 6 after human chorionic gonadotropin to days 11 to 13 after human chorionic gonadotropin, respectively. The serum relaxin concentration increased 8-fold between the 11- to 15-day interval and the 16- to 50-day interval after human chorionic gonadotropin and another 6-fold to the 51- to 90-day interval after human chorionic gonadotropin (all p < 0.01). Multiple linear regression analysis showed that the serum estradiol level 11 to 13 days after human chorionic gonadotropin and the serum human chorionic gonadotropin level 11 to 15 days after human chorionic gonadotropin were the most powerful paired predictors of the concentration of serum relaxin measured in the 11- to 15- day interval after human chorionic gonadotropin interval (R² = 0.39, n = 50), the 16- to 50-day interval (R² = 0.61, n = 51), and the 51- to 90-day interval (R² = 0.55, n = 39). CONCLUSION: Secretion of relaxin is determined by an interaction of the late luteal functional status of the corpora lutea and the human chorionic gonadotropin secreted by the implanting pregnancy. These data allow for the hypothesis that inducing functional luteolysis by substituting one or more injections of luteinizing hormone for the human chorionic gonadotropin injection may decrease secretion of steroids, relaxin, and other factors from the corpora lutea during pregnancy, decreasing the risk of premature delivery in multiple gestations and the ovarian hyperstimulation syndrome.