Humans are one of the few mammalian viviparous species in which pregnancy is extended beyond the luteal phase, the phase during which progesterone is synthesized by the maternal ovary. Instead, it is the fetal placenta that produces progesterone throughout the latter 2 trimesters of human pregnancy. The placenta is developmentally crucial for reproductive success and is the most conspicuous anatomical novelty of placental mammals. However, before it can exert its dual functions as both an endocrine organ and an organ capable of facilitating gas and nutrient exchange, enormous changes must take place within the uterus to not only tolerate the presence of this hemiallogeneic tissue but to also accommodate and support placental development. The most dramatic of these changes is endometrial decidualization, the origin of which coincides in evolutionary history with invasive placentation. This article builds on the observation that the physiological changes that occur during the nonpregnant secretory phase of the uterine cycle in women are remarkably similar to that seen during pregnancy. The fundamental characteristics of human pregnancy (including endometrial decidualization followed several months later by intrauterine inflammation, uterine contractions, and discharge of the decidual lining from the uterine cavity) are present already in the nonpregnant menstrual cycle and are thus independent of the fetus. We hypothesize that many of the physiological defects that lead to complications during pregnancy and parturition are detectable already during spontaneous decidualization in the nonpregnant state and at the onset of menstruation, and can thus be determined before the onset of pregnancy.
Keywords: decidualization; evolution; menstruation; parturition; placenta; pregnancy.