The success of oocyte donation is influenced by multiple factors, including donor age, embryo quality, and the recipient's endometrial receptivity. Elevated E2
levels have been thought to inhibit implantation in animals and humans [14
]. Oocytes (and thus, the embryos) obtained from young donors may have superior potential compared with oocytes obtained from patients with impaired fertility who are undergoing in vitro fertilization and embryo transfer [2
]. It has been postulated that increased implantation rates in oocyte donation resulted from a more physiologic hormonal milieu that uncoupled endometrial receptivity from oocyte production [16
]. Previous reports comparing natural cycles with ovarian stimulation cycles found a higher incidence of dys-synchrony between endometrial glands and stroma in the ovarian stimulation cycles [17
]. There is evidence of a significant reduction in nuclear receptors for progesterone and estrogen in both gland and stroma after ovarian stimulation. Implantation could be affected if these endometrial responses to ovarian stimulation changed the endometrial morphology and affected the window of receptivity [6
]. In contrast, a large amount of evidence suggests that high levels of estradiol are not detrimental to oocyte quality, fertilization, and embryo cleavage [1
To discern the impact of estradiol levels on folliculogenesis and endometrial receptivity is difficult in conventional in vitro fertilization cycles. However, the oocyte donation model allows to study isolated parameters that may affect outcome, and compare their effects on embryo quality and endometrium receptivity [23
In the present study, the donors were typically young, healthy, not infertile—a more homogeneous cohort than infertile populations. Similarly, endometrial receptivity was relatively constant in the recipients because the uniform artificial preparation, as confirmed by endometrial biopsy in a mock cycle.
As expected, our study demonstrated that greater numbers of oocytes, and therefore greater numbers of embryos, were obtained when donors had higher levels of estradiol. Other studies also indicate that high peak E2
levels are not detrimental to oocyte quality, fertilization, and embryo cleavage; and that to the contrary, elevated E2
levels result in a greater number of oocytes and embryos for selection at the time of embryo transfer or cryopreservation [1
]. These findings are in agreement with observations for high responder (those with peak E2
>2000) undergoing conventional in vitro fertilization. The lesser embryo implantation rates must therefore be imputed to an endometrial effect rather than to egg quality [3
In conclusion, elevated E2 levels in donors were not found to affect pregnancy outcome in oocyte donation cycles. This suggests elevated E2 levels do not compromise oocyte quality or embryo development in vitro, but that elevated E2 levels may diminish endometrial receptivity.