Human Parturition: Nothing More Than a Delayed Menstruation

Abstract

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.

Transcriptome and regulatory maps of decidua-derived stromal cells inform gene discovery in preterm birth

“…Of particular relevance are the roles that decidualized stromal cells play in regulating trophoblast invasion, modulating maternal immune and inflammatory responses at the maternal-fetal interface, and controlling remodeling of the endometrium 47 . In fact, defects in all of these processes have been considered a contributing factor to pregnancy disorders 47,48 . Moreover, we showed that the SNPs in regions…”

The Link Between Human Menstruation and Placental Delivery: A Novel Evolutionary Interpretation

“…For this explanation to be validated, it is necessary to demonstrate that the physiological processes that control menstruation and placenta detachment at birth are similar, if not identical. The approach taken was to examine published papers that describe the mechanisms leading to the arrest of blood supply in the spiral arterioles of the nonpregnant endometrium, and the placental control of maternal endometrial blood flow to the chorionic‐villous region of the placenta upon birth.…”

“…Thus menstruation, as an event related to failed conception, would portend a far more important reproductive process associated with successful delivery of the human fetal placenta. This suggestion is supported by the observation of Pavlicev and Norwitz that menstruation has much in common with parturition, the processes that underlie both being similar. The solution advanced in the present paper goes further by contending that the maternal processes underlying parturition are the important selected phenotype that is induced by the fetus prior to parturition.…”

“…Collectively, the physiological steps and their sequences in Figures and indicate a strong similarity between menstruation and placental detachment. This is in agreement with the view of Pavlicev and Norwitz . Thus menstruation could be considered an alternate expression of the placental detachment process.…”

“…Hypothetical representation of the processes leading to human placental detachment, based partly on Salamonsen, Pavlicev and Norwitz, Riley et al, and Shynlova et al Placental withdrawal by the fetus induces physiological events in the maternal endometrium leading to a deciduous reaction and detachment of the placenta with residual endometrial tissues attached. No dissolution of fetal tissues occurs during this event, preventing potential mixing of maternal and fetal blood of different blood types.…”