The regulation of plasma relaxin levels during human pregnancy.

The factors that determine the circulating levels of relaxin during pregnancy have been investigated by comparing the plasma levels of relaxin throughout pregnancy in women who became pregnant spontaneously (singleton, n = 240) or following superovulation (singleton and multifetal pregnancies (two to ten conceptuses), n = 83). Some of the women with multifetal pregnancies underwent selective fetal reduction to twin pregnancies. Relaxin levels were higher at 7-34 weeks of gestation in singleton pregnancies achieved following superovulation when compared with levels in spontaneously conceived singleton pregnancies (p < 0.05-0.001). In samples obtained between 10 and 12 weeks of gestation (before fetal reduction for the multifetal pregnancies), plasma relaxin levels correlated with fetal number (r = 0.526, P = 0.0001). Reduction in fetal number to a twin pregnancy did not alter relaxin levels. These data suggest that the circulating levels of relaxin throughout pregnancy are determined during the cycle of conception by gonadotrophin stimulation, and within the first 10 weeks of pregnancy by the luteotrophic stimulus from the conceptus. Furthermore, once corpus luteum synthesis of relaxin is established, then reduction in the luteotrophic stimulus does not appear to affect it.