The window for embryo transfer in oocyte donation cycles depends on the duration of progesterone therapy

In 192 oocyte donation cycles performed between January 1993 and July 1996,we examined the width of 'the window for embryo transfer' using standardhormonal replacement methods. All transfers were performed within 48 h ofinsemination. We varied the day of embryo transfer with regard to theinitiation of progesterone therapy and, thus, the duration of endometrialexposure to progesterone and analysed the resulting pregnancy rates.Patients were divided into five groups (I-V) and embryo transfers wereperformed 2, 3, 4, 5 or 6 days following initiation of progesteronetherapy. The number of pregnancies per transfer cycle achieved in groupsI-V were 0 (0%), 3 (12%), 16 (40%), 29 (48.3%), and 10 (20.4%)respectively. The increased pregnancy rate in group III in comparison togroup II is statistically significant (P < 0.03). Furthermore, thepregnancy rate in group IV (5 days of progesterone administration beforeembryo transfer) was significantly higher than in group V (6 days ofprogesterone administration before embryo transfer; P < 0.005). We alsonoted that, when embryos were transferred 4 or 5 days after initiation ofprogesterone therapy, the pregnancy rates were not significantly differentbetween menopausal and cycling recipients (50% vs 43.7%). Our resultsindicate that the window for embryo transfer is dependent on duration oftreatment with progesterone; it begins approximately 48 h after startingprogesterone administration and lasts for approximately 4 days. The optimumperiod for transferring embryos at the 4- to 8-cell stage corresponds tocycle days 18 and 19. Transfers performed on the 17th and 20th days of thecycle can result in successful implantation, although the rates ofimplantation are highest when transfers are done on days 18 and 19.