Cervical weakness, also called cervical incompetence or cervical insufficiency, is a medical condition of pregnancy in which the cervix begins to dilate (widen) and efface (thin) before the pregnancy has reached term. Definitions of cervical weakness vary, but one that is frequently used is the inability of the uterine cervix to retain a pregnancy in the absence of the signs and symptoms of clinical contractions, or labor, or both in the second trimester. Cervical weakness may cause miscarriage or preterm birth during the second and third trimesters. It has been estimated that cervical insufficiency complicates about 1% of pregnancies, and that it is a cause in about 8% of women with second trimester recurrent miscarriages.

A sign of cervical weakness is funneling at the internal orifice of the uterus, which is a dilation of the cervical canal at this location.

In cases of cervical weakness, dilation and effacement of the cervix may occur without pain or uterine contractions. In a normal pregnancy, dilation and effacement occurs in response to uterine contractions. Cervical weakness becomes a problem when the cervix is pushed to open by the growing pressure in the uterus as pregnancy progresses. If the responses are not halted, rupture of the membranes and birth of a premature baby can result.

The older terminology is perceived as blaming the woman for the miscarriage, as if she were an incompetent or insufficient person. Consequently, cervical weakness is the recommended term.

Risk factors for premature birth or stillbirth due to cervical weakness include:

Diagnosis of cervical weakness can be challenging and is based on a history of painless cervical dilation usually after the first trimester without contractions or labor and in the absence of other clear pathology. In addition to history, some providers use assessment of cervical length in second trimester to identify cervical shortening using ultrasound. However, short cervical length has actually been shown to be a marker of preterm birth rather than cervical weakness. Other diagnostic tests that have been suggested which have not been validated include hysterosalpingography and radiographic imaging of balloon traction on the cervix, assessment of the patulous cervix with Hegar or Pratt dilators, the use of a balloon elastance test, and use of graduated cervical dilators to calculate a cervical resistance index.

Normally, the cervix should be at least 30 mm in length. Cervical weakness is variably defined. However, a common definition is a cervical length of less than 25 mm at or before 24 weeks of gestational age. The risk of preterm birth is inversely proportional to cervical length:

Cervical weakness is not generally treated except when it appears to threaten a pregnancy. Cervical weakness can be treated using cervical cerclage, a surgical technique that reinforces the cervical muscle by placing sutures above the opening of the cervix to narrow the cervical canal.