Prolongation of follicular phase by delaying hCG administration results in a higher incidence of endometrial advancement on the day of oocyte retrieval in GnRH antagonist cycles

BACKGROUND: Prolongation of follicular phase by delaying hCG administration has been reported to result in a significantly lower ongoing pregnancy rate that did not seem to be due to an embryonic factor. The aim of this prospective randomized study was to assess the effect of delaying hCG administration on endometrial histology. METHODS: Ten oocyte donors underwent endometrial biopsy on the day of oocyte retrieval and endometrial histology was assessed by Noyes' criteria. Ovarian stimulation was performed with recombinant (r)FSH and daily GnRH antagonist starting on day 6 of stimulation. Patients were randomized by a computer-generated list to receive 10 000 IU of hCG either as soon as ≥3 follicles ≥17 mm were present on ultrasound (early-hCG group, n=5) or 2 days after this criterion was met (late-hCG group, n=5). RESULTS: When hCG was delayed, endometrial advancement was present in all samples examined (median advancement 3 days, range 2–3 days). On the contrary, no secretory changes were observed when the follicular phase was not prolonged (difference in the proportion of patients with advancement between the early-hCG and the late-hCG group: 100%, 95% CI: 38–100). CONCLUSIONS: Prolongation of follicular phase by delaying hCG administration results in a higher incidence of endometrial advancement on the day of oocyte retrieval in GnRH antagonist cycles.