Transient Neonatal Uterine Bleeding Offers a Window to Understanding Uterine Development

The phenomenon of neonatal uterine bleeding was observed over centuries. It was dismissed as having no clinical relevance. Alternatively, it was hypothesized to be associated with early-onset endometriosis, but its exact clinical significance remains unknown. We conducted a narrative review using an updated literature of Medline database to July 2026. We examined all identified articles on NUB and examined recent controversies. We explored the relevance of the phenomenon to early-onset endometriosis. We highlight NUB as a putative marker for endometrial maturation and development. The incidence of NUB varies with gestational age. The occurrence of bleeding originating in the uterus of some neonates is well documented. It is possible that some instances of bleeding may not be uterine in origin. The fetal and neonatal endometrium differ significantly from that of the adult. Thus, uterine bleeding in the neonate is not analogous to menstruation. While the factors affecting fetal endometrial development are largely unknown, the occurrence of bleeding in some, but not all, neonates call for a better understanding of the uterine development and the underlying mechanisms of bleeding. The observation that some, but not all, neonates exhibit bleeding may have implications on future uterine development and function. The hypothesis that NUB is associated with early-onset endometriosis, while plausible, has no proven evidence but is difficult to falsify. Whether or not an association exists between NUB and early-onset endometriosis, the phenomenon warrants focus on the physiological stages of early uterine development and maturation and how these may be affected by in utero and early neonatal events.