PIP: To describe normal relationships between the various plasma unconjugated estrogens and progesterone during the second half of human pregnancy, the plasma concentrations of progesterone, 17-hydroxyprogesterone (17-OHP), and unconjugated estrone (E1), estradiol (E2), and estriol (E3) were measured in 126-310 normal women. Progesterone and unconjugated E1, E2, and E3 increased gradually throughout later pregnancy; 17-OHP increased only after the thirty-third week. At term the mean value of progesterone was 9 times higher than that 17-OHP. Throughout pregnancy the mean value of E2 was higher than that of E1 or E3. During the second half of pregnancy the ratios of progesterone to estradiol and estriol and of estradiol to estriol remained unchanged, indicating no preferential increase of plasma concentration of maternal or fetal hormones.