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 interstitial 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 that is dependent on sex steroid hormone–directed changes in blood flow in the spiral arteries. With nonfertile, but ovulatory, ovarian cycles, menstruation effects desquamation of the endometrium. New endometrial growth and development must be initiated with each cycle, so that endometrial maturation corresponds rather precisely with the next pregnancy (implantation) opportunity. There seems to be a very narrow window of endometrial receptivity to blastocyst implantation in the human that corresponds approximately to menstrual cycle days 20 to 24.
To place repetitive menstruation in perspective, the lifetime cumulative blood loss associated with normal endometrial shedding is 10 to 20 liters or more, an amount of blood that contains at least three times the total body iron content of the average adult woman. The approximately 38-year reproductive lifetime cumulative production of progesterone by corpora lutea and placenta in the woman who has two pregnancies and 450 nonfertile ovarian cycles is about 150,000 mg (150 g), which is similar to the cumulative amount of cortisol secreted by the adrenal cortices during the same 38 years. This incredible investment in endometrial tissue growth provides for regular renewal of the functional portion of this tissue in preparation for the next pregnancy opportunity.