[Endometrial biopsy in the evaluation of endometrial receptivity]

The histological changes underwent by endometrium during a natural menstrual cycle were described more than 50 years ago. Dating yields several methodological flaws, with intra- and inter-observer variability and shows questionable relationship to endometrial receptivity. The best studied endometrial factors involved in implantation include the formation of luminal epithelial "pinopodes", expression of adhesion molecules and of cytokines. Ovarian stimulation for IVF is known to affect luteal phase function. Factors influencing the endometrial receptivity in such cycles are poorly understood. Studies comparing the endometrium in IVF cycles with natural cycle controls have shown premature secretory changes in the postovulatory and early luteal phase of IVF cycles. These findings suggest a profound modification of luteal endometrial development in stimulated cycles. Studies exploring the endometrium within the cycle of embryo transfer have shown a deleterious effect of severe periovulatory maturation advancement exceeding 3 days, as no clinical pregnancies were observed in this condition.