Evaluation of the mechanism for higher pregnancy rates in donor oocyte recipients by comparison of fresh with frozen embryo transfer pregnancy rates in a shared oocyte programme.

The objective of this study was to determine the mechanism for higher pregnancy rates in oocyte recipients by comparing the pregnancy rates following fresh and frozen embryo transfers in a shared oocyte programme. A prospective study was carried out of 135 matched pairs of donors and recipients who equally share the donors' pool of oocytes. Recipients were subclassified by ovarian function: 69 were in ovarian failure and 66 retained ovarian function. A total of 474 standard in-vitro fertilization cycles using the same ovarian stimulation protocol as the donors were also evaluated. The main outcome measures were the clinical pregnancy and implantation rates for donors and recipients following fresh and frozen embryo transfers. The clinical pregnancy rates per transfer for fresh embryo transfers were 17.5% for donors, 20.4% for recipients with ovarian function and 46.3% for recipients in ovarian failure (P < 0.05). The pregnancy rates for frozen embryo transfers were 15.3% for donors, 17.2% for recipients with ovarian function and 23.8% for recipients in ovarian failure (not significantly different). The implantation rates for fresh transfers were 7.5% for donors, 8.6% for recipients with ovarian function and 15.6% for recipients in ovarian failure (P < 0.05); for frozen cycles, the implantation rates were 5.1, 5.2 and 7.1% respectively (not significantly different). When classified by age and ovarian function, the clinical pregnancy rates per transfer for recipients with ovarian function were 14.0% for those aged > or = 40 and 22.2% for those aged < 40 years. For recipients in ovarian failure, the pregnancy rates were 33.3% for the older group of women and 39.4% for the younger group. A logistic regression analysis found that ovarian function was the only factor to have an independent effect on outcome. The demonstration of higher pregnancy and implantation rates in recipients versus donors following fresh embryo transfer, despite the use of a common pool of oocytes, strongly suggests that the well-known higher fecundity found in recipients is not predominantly related to the use of better quality oocytes. The demonstration of an implantation rate twice as high following fresh versus frozen embryo transfer in recipients with ovarian failure suggests that the frozen embryo is not as hardy as the fresh embryo. Thus, the fact that both the pregnancy and implantation rates in donors were the same with fresh versus frozen embryo transfer suggests that the ovarian stimulation regimen has a negative effect on outcome. However, the clear demonstration of higher pregnancy rates in recipients with ovarian failure compared with those with ovarian function suggests that, in addition, these higher rates may be linked to a superior uterine environment in patients with ovarian failure. Alternatively, the use of gonadotrophin-releasing hormone agonists may have a negative effect on implantation in patients with ovarian function.