Ovarian stimulation protocol for in vitro fertilization with gonadotropin-releasing hormone agonist widens the implantation window.

Pregnancy rates vary considerably with the type of ovarian stimulation used for in vitro fertilization and embryo transfer (IVF-ET). The window of implantation may represent one of the rate-limiting steps in IVF success. We therefore investigated estimated implantation times of 10 consecutive IVF singleton pregnancies, achieved using pituitary suppression with gonadotropin-releasing hormone agonist (GnRH-a) before and during ovarian stimulation with human menopausal gonadotropins (hMG), and compared those with 9 consecutive IVF pregnancies achieved by hMG stimulation only. Estimated implantation times were calculated by regression analysis of serial human chorionic gonadotropin (hCG) measurements between days 7 and 16 after ET. The GnRH-a/hMG pregnancies implanted between days 7 and 11, whereas hMG pregnancies implanted between days 7 and 9 after ET. The hCG regression curve for the GnRH-a/hMG pregnancies revealed a delay of 1.5 days in estimated implantation time compared with the hMG only group. There were no significant differences in pretransfer in vitro embryos development between the two groups. Thus, the delay in hCG rise probably reflects a delay in embryo implantation. We therefore conclude that a GnRH-a/hMG stimulation protocol appears to widen the implantation window in comparison with a hMG only protocol. This observation may at least in part explain the improved IVF pregnancy success with GnRH-a/hMG stimulation protocols.