A.J.Duleba2, D.L.Olive2, A.Chatziparasidou1 and G.Vlassis1

3To whom correspondence should be addressed In 192 oocyte donation cycles performed between January 1993 and July 1996, we examined the width of ‘the window for embryo transfer ’ using standard hormonal replacement methods. All transfers were performed within 48 h of insemination. We varied the day of embryo transfer with regard to the initiation of progesterone therapy and, thus, the duration of endometrial exposure to progesterone and analysed the resulting pregnancy rates. Patients were divided into five groups (I–V) and embryo transfers were performed 2, 3, 4, 5 or 6 days following initiation of progesterone therapy. The number of pregnancies per transfer cycle achieved in groups I–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 to group II is statistically significant (P, 0.03). Further-more, the pregnancy rate in group IV (5 days of progester-one administration before embryo transfer) was significantly higher than in group V (6 days of progesterone administration before embryo transfer; P, 0.005). We also noted that, when embryos were transferred 4 or 5 days after initiation of progesterone therapy, the pregnancy rates were not significantly different between menopausal and cycling recipients (50 % vs 43.7%). Our results indicate that the window for embryo transfer is dependent on duration of treatment with progesterone; it begins approxi-mately 48 h after starting progesterone administration and lasts for approximately 4 days. The optimum period for transferring embryos at the 4- to 8-cell stage corresponds to cycle days 18 and 19. Transfers performed on the 17th and 20th days of the cycle can result in successful implantation, although the rates of implantation are highest when transfers are done on days 18 and 19. Key words: embryo transfer window/implantation/menopausal recipients/oocyte donation/replacement therap