Genital tract outflow is important for the expulsion of normal secretions from the cervix and vagina. Outflow is also critical for menstrual efflux. Outflow obstruction may occur at different levels with resultant variations in clinical presentation. With appropriate treatment, the prognosis is excellent, and full recovery is typical.
Embryologically, the lower two thirds of the vagina develops from the urogenital sinus. The upper vagina, cervix, uterus, fallopian tubes, and ovaries form from the müllerian duct system. Failure of vertical fusion or canalization of the two systems in utero may result in bicornuate uterus, uterus didelphus, cervical stenosis or atresia, vaginal atresia, or longitudinal or transverse vaginal septa. In addition, hymenal tissue may be imperforate.
The exact etiology of such genital tract anomalies is unclear. Any defect in the normal organogenesis involving the urogenital sinus or the müllerian duct can result in genital tract anomalies. Interest has been focused on abnormalities in the structure; expression or function of the mammalian HOX genes may be a possible etiology for such malformations. 
Among patients who have posttransplant graft versus host reactions, scarring of the vagina and vaginal agglutination can result in acquired outflow obstructions similar to those seen in patients with lichen planus lesions of the vagina.
Female genital malformations occur in 5-7% of the general population. [2, 3] The incidence of müllerian agenesis is about one in 4500 women; 30-36% of such malformations are associated with other anomalies as well, particularly anomalies of the kidneys and skeleton. Imperforate hymen is the most common genital outflow tract anomaly.
Although imperforate hymen is a congenital anomaly and can be detected early in life, imperforate hymen remains undetected until puberty in many patients. At birth, the presence of increased mucous secretions in the vagina secondary to maternal estrogen effects may result in a mucocele appearing as a bulging hymen, which is easily observed.
If not detected at birth, secretions resorb, and the condition may be undetected until puberty, when menstrual blood collects and causes hematocolpos, a bluish-colored bulging hymen. Patients with undiagnosed imperforate hymen who have reached menarche may have recurrent, cyclic, lower abdominal pain.