Vaginal outlet anomalies should be considered in infants presenting with an abdominal mass or in adolescents with abdominal pain, particularly if the patient has normal pubertal development without the onset of expected menarche Vaginal outlet obstruction can cause acute urinary retention in any age female A complete physical examination can easily diagnose imperforate hymen CONGENITAL VAGINAL OBSTRUCTION An obstructed vagina will eventually accumulate fluid, causing distention and, eventually, symptoms Anatomic vaginal obstructions present most often during adolescence, when menstrual blood fills the vagina, producing hematocolpos or hematometrocolpos There are multiple case reports of vaginal obstruction presenting in the neonatal period (incidence of approximately 0.006%), when vaginal distention is caused by mucus secreted as a result of stimulation by maternal hormones, a condition called hydrometrocolpos The two most common anomalies that lead to vaginal outlet obstruction are imperforate hymen (approximately 0.1% of term female neonates) and transverse vaginal septum (sometimes called vaginal atresia; in 30,000 to 84,000 women) These malformations are probably produced between the 16th and 20th weeks’ gestation if the developing vaginal plate fails to perforate at its junction with either the fused paramesonephric (müllerian) ducts proximally or the urogenital sinus caudally Unlike patients with a transverse vaginal septum or imperforate hymen, patients with complete agenesis of the vagina (Rokitansky–Küster– Hauser syndrome) have rudimentary uteri or none at all, so hydrocolpos does not occur Although vaginal obstruction should be properly identified during the initial examination of the newborn female, infants with hydrocolpos often go unrecognized until days or weeks later when they develop the three hallmarks of this condition: (i) a lower abdominal mass, (ii) difficulty with urination, and (iii) a visible bulging membrane at the introitus In more severe cases, infants may also have constipation, hydronephrosis, edema of the lower extremities, and hypoventilation A complete physical examination should immediately indicate the proper diagnosis In adolescence, a female with congenital vaginal obstruction will usually present with primary amenorrhea and/or lower abdominal pain with associated hematocolpos; this condition should be suspected in patients with