Loss of a conceptus early in development can be detected by very sensitive assays specific for hCG. We examined 20 menstrual cycles ending in early loss of a conceptus in order to identify hormonal correlates of loss. Each loss cycle was compared to a successful conception cycle in the same woman, using daily concentration of urinary estrone-3-glucuronide and pregnanediol-3-glucuronide (PdG). The estrone-3-glucuronide and PdG profiles in cycles of early pregnancy loss were very similar to those in successful conception cycles until late in the luteal phase. Early pregnancy loss was not related to a midluteal deficiency in PdG. hCG tended to be detected later in cycles of early pregnancy loss than in successful conception cycles, presumably indicating later implantation. Ten of the early pregnancy losses implanted after luteal-day-10; only one of the successful pregnancies implanted that late. The corpus luteum responded to the conception in only 2 of the 10 loss cycles with late implantation. In contrast, the corpus luteum responded in 8 of 10 loss cycles with normally timed implantation. The similarity of preimplantation hormonal profiles in cycles of early pregnancy loss and in cycles with successful conceptions suggests that most early losses in reproductively normal women do not result directly from deficiencies in ovarian steroid production.