All obstetricians should be aware of the basic reproductive biological processes required for women to successfully achieve pregnancy. Several abnormalities can affect each of these 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. Moreover, these hormones promote optimal endometrial regeneration after menstruation in preparation for the next implantation window.
Should fertilization occur, events that begin after initial blastocyst implantation onto the endometrium and continue 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 initially maintains pregnancy and eventually initiates events leading to parturition. The following sections address the physiology of the ovarian-endometrial cycle, implantation, placenta, and fetal membranes, as well as specialized endocrine arrangements between fetus and mother.
The Ovarian–Endometrial Cycle
Predictable, regular, cyclical, and spontaneous ovulatory menstrual cycles are regulated by complex interactions of the hypothalamic-pituitary axis, ovaries, and genital tract (Fig. 5-1). The average cycle duration is approximately 28 days, with a range of 25 to 32 days. The hormonal sequence leading to ovulation directs this cycle. Concurrently, cyclical changes in endometrial histology are faithfully reproduced. Rock and Bartlett (1937) first suggested that endometrial histological features were sufficiently characteristic to permit cycle “dating.” In this scheme, the follicular-proliferative phase and the postovulatory luteal-secretory phase are customarily divided into early and late stages. These changes are detailed in Chapter 15 of Williams Gynecology, 2nd edition (Halvorson, 2012).
Gonadotropin control of the ovarian and endometrial cycles. The ovarian-endometrial cycle has been structured as a 28-day cycle. The follicular phase (days 1 to 14) is characterized by rising estrogen levels, endometrial thickening, and selection of the dominant “ovulatory” follicle. During the luteal phase (days 14 to 21), the corpus luteum (CL) produces estrogen and progesterone, which prepare the endometrium for implantation. If implantation occurs, the developing blastocyst begins to produce human chorionic gonadotropin (hCG) and rescues the corpus luteum, thus maintaining progesterone production. FSH = follicle-stimulating hormone; LH = luteinizing hormone.
Follicular or Preovulatory Ovarian Phase
The human ovary contains 2 million oocytes at birth, and approximately 400,000 follicles are present at puberty onset (Baker, ...