Neonatal menstruation explains epidemiological links between fetomaternal conditions and adolescent endometriosis

Background: Different fetomaternal conditions may influence the risk of endometriosis during adolescence and in adult life; here we focus on the hormonal maturation of the fetal endometrium in the final stages of pregnancy and on the theory that neonatal menstruation should be considered, similar to cyclic menstruation in the adult, as a risk factor for adolescent endometriosis. Methods: The literature on neonatal menstruation and associated factors was systematically searched, and 19 relevant articles, published in different languages between 1950 and 1984, were retrieved. After closer scrutiny, 11 publications were selected as relevant. Results: At birth, the neonatal endometrium displays different degrees of progesterone resistance, varying from a complete absence of progesterone responses, to secretory activity, decidualization and menstrual-like shedding. A temporal relationship exists between endometrial maturation and the incidence of neonatal menstruation, supporting the hypothesis that vaginal bleeding at birth is triggered by progesterone withdrawal. Neonatal menstruation occurs rarely in preterm babies, increases in those born at term and is a relatively frequent event in postmature infants. Analysis of archival clinical studies indicates that being born postterm or to a preeclamptic mother increases the risk of neonatal menstruation. Low birthweight may also enhance the likelihood of neonatal menstruation, whereas prematurity could be protective, although the available data are inconclusive. Conclusions: The available data suggest that fetomaternal risk factors associated with neonatal menstruation could also potentially be useful in identifying women at risk of endometriosis. However, archival clinical studies have important limitations, including lack of accurate dating of pregnancy, therefore necessitating prospective studies and systematic registration of neonatal menstruation.