The endometrium in stimulated cycles for IVF.

Ovarian stimulation for IVF is known to affect luteal phase function. The endometrium in IVF cycles is thus subject to an altered endocrinological environment and to a possible direct effect of the ovarian stimulation therapy. Factors influencing the endometrial receptivity in such cycles are poorly understood. Studies comparing the endometrium in IVF cycles with natural cycles as controls have shown premature secretory changes in the post-ovulatory and early luteal phase of IVF cycles, followed by a large proportion of dyssynchronous glandular and stromal differentiation in the mid-luteal phase. These findings suggest a profound modification of luteal endometrial development in stimulated cycles. This hypothesis is further supported by the demonstration of a modified endometrial steroid receptor regulation and a profound antiproliferative effect in IVF cycles. The time of maximal endometrial receptivity is defined as the implantation window and is characterized by the expression of various endometrial products, among which pinopodes, integrins and leukaemia inhibitory factor are best described. Premature expression of pinopodes and integrins are in line with the observation of precocious luteal transformation following ovarian stimulation, although the clinical relevance with respect to the establishment of a clinical pregnancy awaits further validation. Studies exploring the endometrium within the cycle of embryo transfer have shown a deleterious effect of severe peri-ovulatory maturation advancement exceeding 3 days, as no clinical pregnancies were obtained in this condition. Further unravelling of molecules involved in the implantation mechanism is needed for a better comprehension of the link between altered endometrial development and receptivity in IVF cycles.