Imperforate hymen

The hymen is a thin membrane that covers part of the opening of the vagina. It usually has perforations that allow the vagina to communicate with the outside, allowing the drainage of vaginal secretions and menstruation. 
It is called imperforate hymen when it covers the entire opening of the vagina and has no holes, causing an obstruction of the vaginal fluids. 
Imperforate hymen is a rare congenital malformation that occurs in 0.1% of newborn girls. 

It usually does not cause symptoms until the first menstrual period, or even a few years later. As menstrual periods occur, blood accumulates in the uterine cavity (haematometra), or in the vagina (haematocolpos) until it causes severe pelvic pain, abdominal distension and symptoms due to compression of pelvic organs, such as constipation or urinary retention. The most characteristic presentation is cyclic abdominal pain in adolescents who have not yet had their first menstrual period.

Diagnosis is clinical and is based on questioning of symptoms and physical examination where an imperforate hymen is observed. Abdominal ultrasound may detect fluid in the vagina, suggestive of haematocolpos.

Treatment is surgical, with a star-shaped incision being made in the hymen to drain the material accumulated in the vagina.