Women with incompetent cervix experience preterm deliveries because their cervix is either too short or too weak to sustain a full-term pregnancy. A woman’s cervix should open at the beginning of labor after about nine months of pregnancy. Yet, in these women, pressure from the growing fetus in the uterus causes the cervix to open prematurely, leading to preterm delivery in the second trimester. The pregnancy loss typically occurs between the 16th and 24th weeks of pregnancy — most commonly between weeks 18 and 22.
Experts have not identified an exact cause or risk factors that lead to cervical weakness in women with incompetent cervix. Though it rarely occurs, incompetent cervix can develop after a woman has already carried one or more successful pregnancies.