The serum relaxin and human chorionic gonadotropin (hCG) concentrations were determined in 17 spontaneous singleton pregnancies and four singleton pregnancies and five multiple gestations that followed treatment with menotropins. The geometric mean serum relaxin concentrations for the control and postmenotropins singleton pregnancies were similar (both 1.13 ng/mL), but the geometric mean serum relaxin concentration for multiple gestations that followed treatment with menotropins was significantly higher (2.56 ng/mL, P<.01). A significant regression of serum relaxin on either days from the last menstrual period or serum hCG concentration was demonstrated. Covariance analysis supported the conclusion that multiple gestation produced a significant increase in serum relaxin concentration.