THE HORMONAL-CONTROL OF ENDOMETRIAL RECEPTIVITY - ESTROGEN (E-2) AND PROGESTERONE

While the number of identified substances produced by the ovary increa ses steadily, it remains remarkable that the sole use of exogenous est rogen (E-2) and progesterone (P) can prime optimal endometrial recepti vity in women whose ovaries have failed or are absent. Early work show ed that a marked leeway existed in the acceptable duration of the E-2- only phase of endometrial priming. Subsequently, a sequence of transfo rmations are induced by exogenous progesterone that reproduces classic al findings made in the menstrual cycle, Secretory changes in endometr ial glands are best seen between the 4th and 6th day of progesterone a dministration (day 18-20 of an ideal cycle where progesterone exposure starts on day 15), Predecidual changes of the endometrial stroma are apparent starting on the 10th day of progesterone exposure (day 24). C ontrary to earlier belief, even maximal alterations in the plasma E-2 to progesterone ratio fails to alter the endometrial morphology of eit her glands or stroma. More recently it has been recognized that E-2 an d progesterone also affect uterine contractility. It has been postulat ed that excessively high levels of E-2 may increase uterine contractil ity and adversely affect implantation rates in in-vitro fertilization (IVF). Exogenous progesterone has been shown to exert utero-relaxing e ffects and it has been hypothesised that progesterone supplementation before embryo transfer (ET) may improve receptivity in IVF. (C) 1998 E lsevier Science Ireland Ltd. All rights reserved.