RADIOLOGY: ABDOMEN: Case# 33657: HYDROCOLPOS. Newborn infant girl with a pelvic mass. Trans perineal and abdominal ultrasound images reveal a homogeneously echogenic tubular structure posterior to the bladder within the pelvis. Obstruction of the vagina results in hydrocolpos or hydrometrocolpos and accounts for approximately 15% of pelvic masses in newborn infant girls. Obstruction occurs secondary to an imperforate hymen or an atretic or stenotic vagina. Secretions or blood will accumulate and cause dilatation of the obstructed vagina. There is a high incidence of genitourinary, gastrointestinal, and skeletal anomalies associated with vaginal atresia. A simple imperforate hymen may not be associated with other anomalies.