All obstetricians should be aware of the basic reproductive biological processes required for women to successfully achieve pregnancy. A number of abnormalities can affect each of these processes and lead to infertility or pregnancy loss. In most women, spontaneous, cyclical ovulation at 25- to 35-day intervals continues during almost 40 years between menarche and menopause. Without contraception, there are approximately 400 opportunities for pregnancy, which may occur with intercourse on any of 1200 days—the day of ovulation and its two preceding days. This narrow window for fertilization is controlled by tightly regulated production of ovarian steroids. These hormones promote optimal regeneration of endometrium after menstruation ends in preparation for the next implantation window.
Should fertilization occur, the events that unfold after initial implantation of the blastocyst onto the endometrium and through to parturition result from a unique interaction between fetal trophoblasts and maternal endometrium-decidua. The ability of mother and fetus to coexist as two distinct immunological systems results from endocrine, paracrine, and immunological modification of fetal and maternal tissues in a manner not seen elsewhere. The placenta mediates a unique fetal–maternal communication system, which creates a hormonal environment that helps initially to maintain pregnancy and eventually initiates the events leading to parturition. The following sections address the physiology of the ovarian-endometrial cycle, implantation, placenta, and fetal membranes, and specialized endocrine arrangements between fetus and mother.
The endometrium-decidua is the anatomical site of blastocyst apposition, implantation, and placental development. From an evolutionary perspective, the human endometrium is highly developed to accommodate endometrial implantation and a hemochorial type of placentation. Endometrial development of a magnitude similar to that observed in women—that is, with special spiral (or coiling) arteries—is restricted to only a few primates, such as humans, great apes, and Old World monkeys. Trophoblasts of the blastocyst invade these endometrial arteries during implantation and placentation to establish uteroplacental vessels.
These primates are the only mammals that menstruate, which is a process of endometrial tissue shedding with hemorrhage and is dependent on sex steroid hormone-directed changes in blood flow in the spiral arteries. With nonfertile, but ovulatory, ovarian cycles, menstruation effects endometrial desquamation. New growth and development must be initiated with each cycle so that endometrial maturation corresponds rather precisely with the next opportunity for implantation and pregnancy. There seems to be a narrow window of endometrial receptivity to blastocyst implantation that corresponds approximately to menstrual cycle days 20 to 24.
The development of predictable, regular, cyclical, and spontaneous ovulatory menstrual cycles is regulated by complex interactions of the hypothalamic-pituitary axis, the ovaries, and the genital tract (Fig. 3-1). The average cycle duration is approximately 28 days, with a range of 25 to 32 days. The sequence of hormonal events leading to ovulation directs the menstrual cycle. The cyclical changes in endometrial histology are faithfully reproduced during each ovulatory cycle.