The endometrium from the neonate to the adolescent

Scientific data on the phenomenon of uterine bleeding in the neonate (NUB) began to appear over a century ago when angiogenesis and petechial haemorrhages in the endometrium of newborn female infants, as well as fluid blood within the uterine cavity, were first described.

A thorough search of the 20th century literature was carried out to identify studies reporting data on NUB.

The neonatal endometrium, although not identical to the adult, shows cellular responses of the same type, and at birth the endometrium is proliferative in 2/3 of all newborns with the residual 1/3 showing secretory, decidual or menstrual features. In the latter neonates, the presence of a functional cervical obstruction may expose them to menstrual regurgitation including neonatal mesenchymal stem-like cells. This can represent the origin of early-onset endometriosis. At menarche, partial or full ontogenic progesterone resistance can occur in the majority. If persisting, adolescent pregnancy may be exposed to major obstetrical syndromes including preeclampsia and fetal growth restriction.

Increasing evidence indicates that the origin of major reproductive disorders in adolescents may lie in the degree of progesterone response in the neonate. The spectrum of progesterone response from resistance till decidualisation may explain the early occurrence of major obstetrical syndromes and endometriosis, respectively.