Endometrial preparation: lessons from oocyte donation.

OBJECTIVE: To gain insight into the physiology of human endometrial development after artificial preparation with estrogen (E) and P, before oocyte donation.
DESIGN: Review and analysis of relevant studies published in the last decade, identified through the literature and Medline searches.
RESULTS: Oocyte donation represents a unique in vivo experimental model in the human that permits the study of endometrial development under controlled variable conditions. Early studies have shown that adequate endometrial preparation can be achieved by sequential E and P only. The successful implementation of the simplified approach to oocyte donation demonstrated that satisfactory endometrial receptivity is not dependent on incremental administration of E and P and similarly can be achieved by fixed dosages of these steroids. Moreover, numerous clinical oocyte donation studies have shown that both physiologic and supraphysiologic levels of E and P have resulted in good endometrial development and pregnancy rates, underlining the relative insensitivity of the endometrium to extreme hormonal conditions. In addition, it has been clarified that the endometrium is tolerant of some manipulations during the follicular phase. Contrary to morphological studies that demonstrated preservation of endometrial preparation after luteal E depletion, preliminary evidence suggests that the functional capacity of the endometrium could be affected adversely.
CONCLUSION: In contrast to early oocyte donation studies, which indicated a correlation between morphologic integrity and functional capacity of the endometrium, some evidence presented in this review demonstrates that adequate endometrial morphology does not always imply normal endometrial receptivity.