The effect of cycle length on the outcome of in vitro fertilization

In order to study the effect of cycle length on the pregnancy rate in an in vitro fertilization and embryo transfer (IVF-ET) program, 173 consecutive patients were divided into short menstrual cycle (mode 26 days or less) and normal cycle (mode 27 days or more) groups. Patients were randomly allocated to one of two treatments, commencing ovarian stimulation with human menopausal gonadotropin (hMG) on either day 2 or day 4 of their cycle. The number of oocytes retrieved and embryos transferred did not differ significantly. The amount of hMG used and day of human chorionic gonadotrophin administration both differed significantly (P less than 0.01) between regimens but was independent of cycle length. Both the clinical pregnancy rate (30.2% versus 9.4%, P less than 0.05) and the number of cleaved embryos giving rise to gestation sacs (16% versus 3.4%, P less than 0.02) was significantly higher in patients with a normal cycle length. Mode cycle length has a significant bearing on the outcome of IVF-ET cycles.