The hymen is the thin tissue around the entry of the vagina that usually has an opening for menstrual blood to pass through. Imperforate hymen means that the hymen does not have an opening, so the entry of the vagina is completely closed off by the hymen.
Imperforate hymen is often discovered when a girl starts her period. Because menstrual blood cannot get out of the vagina, the blood backs up. This causes pain and pressure in the pelvic area.
Imperforate hymen is treated with surgery to create an opening in the hymen.
WebMD Medical Reference from Healthwise