An incompetent cervix is a condition that causes the opening of the lower part of the uterus (cervix) to open (dilate) too early during pregnancy. This happens before the pregnancy reaches early term (week 37) and before labour begins.

Normally, the cervix stays tightly closed as the weight of the baby and the contents of the uterus increase. If the cervix is incompetent, the weight of the fetus and amniotic fluid cause it to open (dilate) too early, which may cause preterm labour.

An incompetent cervix may result from:

• Damage to the cervix after a small section was surgically removed for testing (cone biopsy) or after a loop electrosurgical excision procedure (LEEP).
• Surgical scraping of the inside of the uterus (dilation and curettage, or D&C).
• Damage to the cervix from a previous difficult birth.
• A birth defect of the mother that caused her to have an abnormal cervix.
• Unknown reasons.

Cerclage, a procedure in which the cervix is sewn shut, has been known to allow pregnancy to continue until it is safe for the baby to be born. But it is rarely done because it is hard to know which women will benefit from the procedure. In rare cases, cerclage can cause infection.