In 22 consecutive in vitro fertilization cycles stimulated with purified follicle-stimulating hormone, human chorionic gonadotropin (hCG), estradiol (E2), and progesterone (P) were measured every 3 days during the luteal phase. All serum measurements were normalized to the day of hCG administration (day 0). There was a total of nine pregnancies; two were biochemical pregnancies, whereas 7 of the 22 women had clinical pregnancies (31.8%). Of these, two miscarried and five had term pregnancies (three singleton, two twin). Conception cycles could be differentiated from nonconception cycles by serum E2 levels on day 8 (P = 0.035), by hCG levels on day 11 (P = 0.03), and by P levels on day 14 (P = 0.001). From days 8 to 11, hCG levels plateaued in conception cycles and decreased in nonconception cycles. However, during that period, E2 and P fell in both groups of women. This decline in sex steroids, which was observed in both conception and nonconception cycles, may well negatively influence endometrial development during the peri-implantation period and compromise conception, resulting in failure to conceive, biochemical pregnancy, and early miscarriage.