Abstract
Introduction: Neonatal menstruation is considered the most neglected uterine bleeding, but it was concluded to be a marker of fetal stress. It is more common in preeclampsia, fetal growth restriction, and isoimmunization.¹ It has a prevalence of 5% in healthy newborns, which is lower in preterm newborns and higher in low birth weight newborns.²

Case description: A 6-days-old female born with 41 weeks of gestation presented a 2-day history of vaginal bleeding without other symptoms or history of trauma. Physical examination evidenced vaginal bleeding, without external lesions (fig.1). Complementary diagnostic tests were not performed. Following vigilance, vaginal bleeding was present for 6 days and stopped on the 10th day of life.

Discussion: Vaginal bleeding in the neonatal period results from desquamation of the endometrium, triggered by the decline in circulating steroid hormones. It often begins 3 to 5 days after birth and typically lasts up to 10 days³ - a chronology similar to the case presented. Some newborns with neonatal menstruation experience retrograde menstruation, which causes implantation of endometrial cells in the abdominal cavity, which may lead to an increased risk of early endometriosis. Newborns who have this complication should be evaluated so that an early diagnosis of endometriosis can be made in the future in children who begin with pelvic pain or severe menstrual pain.

Key words: Endometriosis; neonatal period; neonatal uterine bleeding