A high incidence of premature labor, incompetent cervix and fetal wastage occurs in multiple gestations which follow treatment with human menopausal gonadotropins (HMG). In order to determine the effect of treatment with HMG on hormone secretion in human pregnancy, progesterone (PROG), 17 beta-estradiol (E2), estriol (E3) and human chorionic gonadotropin (hCG) were determined by radioimmunoassay in 341 serum specimens from 229 normal singleton pregnancies and in 79 serum specimens from 20 pregnancies following induction of ovulation with HMG in women with either hypothalamic amenorrhea (HA) or the polycystic ovary syndrome (PCO). Fitting equations were found for the log transformed normal values and the residuals were obtained by subtraction of the predicted normal values from the log transformed values observed in the HMG pregnancies. In pregnancies which followed treatment with HMG, PROG and E2 were initially elevated above normal. As pregnancy progressed, the deviation from normal became proportionately less. PROG (P less than 0.025) was lower and E2 (P less than 0.025) and E3 (P less than 0.05) were higher in PCO pregnancies than in HA pregnancies. Multiple gestation produced increases in PROG (P less than 0.005), E2 (P less than 0.005) and E3 (P less than 0.001) in comparison to singleton pregnancies.