Simultaneous measurement of serum concentrations of estrone (E1), estradiol (E2), estriol (E3), and progesterone were carried out in multiple serial blood samples obtained during the last 3–10 weeks of pregnancy, labor, and the immediate postpartum period in 5 normal women. Estrogen and progesterone levels showed a small, but statistically significant diurnal variation during pregnancy. They did not change during labor; however, with the exception of E1 levels, all declined following delivery. Individual patterns preceding labor, derived from calculated moving mean values, showed no consistent decline in progesterone levels nor a surge in E1 and E2 concentrations whereas estriol levels showed a steady rise starting 14–28 days prior to the onset of labor. Whether this E3 elevation reflects fetal maturation and/or plays a role in the triggering mechanism of labor is unknown. Failure to detect changes in E1, E2, and progesterone levels in the maternal peripheral circulation does not preclude the possibility that alterations of metabolism of these hormones in the fetal or uterine compartments might be involved in the initiation of human labor.