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Almost immediately after the implantation of the ovum, its trophoblast begins to proliferate and invade the surrounding decidual tissue. As it does so, it breaks through the walls of the maternal capillaries, from which the blood escapes and forms cavities, which are bounded partly by trophoblast and partly by decidua. The maternal blood spaces established in this manner represent the earliest stages of the intervillous blood spaces of the future placenta.
—J. Whitridge Williams (1903)
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In 1903, the histopathological and embryological descriptions of ovum implantation and placental development had been extensively studied and described. However, the origins and functions of pregnancy hormones were largely unknown. Indeed, it was another 25 to 30 years before estrogen and progesterone were discovered. In the past 50 years, remarkable strides have followed to uncover the steps of implantation and placental structure and function.
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All obstetricians should understand the basic biological steps 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 continues during almost 40 years between menarche and menopause. Without contraception, there are approximately 400 opportunities for pregnancy, namely, the day of ovulation and its few 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.
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If fertilization occurs, events that begin after blastocyst implantation persist until parturition. These derive from a unique interaction between fetal trophoblasts and the maternal endometrium, which has been transformed into the decidua. The ability of a mother and her 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. In addition, 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.
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OVARIAN–ENDOMETRIAL CYCLE
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Predictable, regular, cyclical, and spontaneous ovulatory menstrual cycles are regulated by complex interactions of the hypothalamic-pituitary-ovarian axis. Concurrently, cyclical changes in endometrial histology are faithfully reproduced (Fig. 5-1). Essentials players in this process include follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which are pituitary-derived gonadotropins, and the ovarian sex steroid hormones estrogen and progesterone.
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