The MOST common cause of vaginal bleeding in a neonate is:

Neonatal vaginal bleeding is caused by maternal hormone exposure during pregnancy. These maternal hormones are abruptly withdrawn at the time of bih. It is entirely normal and presents soon after bih as a 'mini period' in females. It will resolve, usually lasting less than a day, and is painless. Ref: A Practical Guide to Pediatric Emergency Medicine: Caring for Children in ... edited by N. Ewen Amieva-Wang, 2011, Page 333 ; Shaw, 13th ed, Page 48