MRI features of hematocolpos in an adolescent with uterus didelphys (Müllerian duct anomaly class III).

Hematocolpos is defined as a blood-filled dilated vagina due to an accumulation of menstrual blood in the setting of an anatomical obstruction, usually an imperforate hymen.

Other causes include vaginal septum including a transverse vaginal septum which is seen in 75% of cases of uterus didelphys.