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Synchronization of the availability of good quality oocytes from donors and adequate endometrial maturation of recipients are very important for the success of an oocyte donation programme. A flexible protocol for the endometrial preparation of recipients is important in timing embryo transfer between days 17 and 19 of the cycle ('window of receptivity'). The purpose of this study was to evaluate the effect of the length of oestradiol administration to recipients on pregnancy outcome. Oestrogen administration was 8 mg/day, but its length varied prospectively from 6 to 27 days, followed by the addition of progesterone (100 mg daily i.m.) for 2-4 days according to the availability of good quality oocytes. Pregnancy outcome was evaluated regardless of age, indication for oocyte donation or number of embryos transferred per patient. The pregnancy rate per cycle was comparable when oestradiol was administered from 6 to 11 days before progesterone addition, while it dropped significantly thereafter. The variation in progesterone administration did not affect pregnancy outcome. These findings provide us with a greater flexibility by allowing us to vary oestradiol administration to recipients from 6 to 11 days prior to progesterone, reducing considerably, therefore, the need to cancel embryo transfer because of oocyte unavailability. Thus we can arrange to transfer embryos between days 17 and 19 of the recipient's cycle so as to obtain the best possible clinical outcome.

The progesterone antagonist mifepristone (RU486), was given in mice once on different days of pregnant mare's serum gonadotrophin-human chorionic gonadotrophin (PMSG-HCG) treatment and its action upon the induction of ovulation studied. RU486 administered on the day after PMSG significantly reduced the ovulation rate. Ovulation was completely inhibited when the progesterone antagonist was given simultaneously with HCG, but RU486 administered 4 h after HCG treatment remained ineffective. The development of two-cell zygotes harvested on day 2 post-coitum from mice treated with RU486 on the day after the PMSG treatment was followed in vitro and showed a significant decrease in the number of embryos developing to blastocysts. These results favour the involvement of progesterone in the ovulation process, indicating a direct effect of this hormone at the ovarian level via a progesterone receptor-mediated action.