The Endometrium in Human Assisted Reproduction (original) (raw)

1991, Annals of the New York Academy of Sciences

The assessment of endometrial function in physiological conditions is one of the most controversial areas in reproduction, and the situation is further complicated in human assisted reproduction by the multitude of variables associated with any form of treatment. So-called medically assisted reproduction implies different degrees of intervention, which are intended to overcome an identified or even a non-identified factor of reproductive failure, but these interventions may potentially alter endometrial receptivity for adequate implantation. Thus, when we select an infertile female patient with evident ovarian function, either normal or abnormal, for a specific assisted reproductive treatment, the first "manipulation" we do, to improve the chance of establishing pregnancy, is controlled ovarian hyperstimulation: a key and critical step between the selection of patients and the adoption of one of the treatment techniques (FIG. 1). Different strategies are adopted for inducing multiple follicular growth. The protocols mainly used are the combination of clomiphene citrate (CC) with human menopausal gonadotropins (hMG), the combination of hMG with gonadotropin releasing hormone agonists (GnRH-a) or hMG alone. Whichever protocol is used, the expected multiple follicular growth will produce supraphysiological levels of estradiol during the follicular phase and the resulting multiple corpora lutea will produce supraphysiological levels of estradiol and progesterone during the luteal phase. Another important aspect regarding the endometrial environment, which differentiates in virro fertilization with intrauterine embryo transfer from other techniques and natural conception, is the different timing of arrival of embryos into the uterine cavity and the different developmental stage of the embryo at that time. In fact, in spontaneous conception, and we have to suppose the same situation exists after the replacement of embryos or gametes into the tubes, a morula or an early blastocyst will arrive into the uterine cavity 5-6 days after the ovulation. In in vitro fertilization (IVF) embryos at 4-8 blastomeric stage are usually replaced into the uterine cavity 48 hours after the oocyte retrieval. Most studies designed to investigate endometrial function have been conducted in IVF and embryo transfer (ET) studies, so we have to keep in mind these points in reviewing this topic.