Hydrocolpos is the distension of the vagina caused by accumulation of fluid due to congenital vaginal obstruction. The obstruction is often caused by an imperforate hymen or less commonly a transverse vaginal septum. The fluid consists of cervical and endometrial mucus or in rare instances urine accumulated through a vesicovaginal fistula proximal to the obstruction. In some cases it is associated with Bardet-Biedl Syndrome. If it occurs in prepubertal girls, it may show up as abdominal swelling. It may be detected by using ultrasound. It may also present at birth as a distended lower abdomen and vagina. It also associated with vaginal atresia.

Vaginal hypoplasia is the underdevelopment or incomplete development of the vagina. Vaginal hypoplasia can vary in severity from being smaller than normal to being completely absent. The absence of a vagina is a result of vaginal agenesis. Diagnostically, it may look similar to a vaginal obstruction. It is frequently associated with Mayer-Rokitansky-Küstner-Hauser (MRKH) syndrome, in which the most common result is an absent uterus in conjunction with a deformed or missing vagina, despite the presence of normal ovaries and normal external genitalia. It is also associated with cervical agenesis, in which the uterus is present but the uterine cervix is absent.

A vaginal obstruction is often caused by an imperforate hymen or, less commonly, a transverse vaginal septum. A sign of vaginal obstruction is hydrocolpos, that is, accumulation of watery fluid within the vagina. It may extend to become "hydrometrocolpos", that is, accumulation of watery fluid within the vagina as well as within the uterus.