Sometimes premature effacement (shortening of the vaginal portion of the cervix and thinning of the walls) and dilation of the cervix is not caused by labor, but rather by structural weakness in the cervix itself. This is called cervical incompetence.
The weakness can result from a number of conditions, most due to prior injury to the cervix or resulting from an inherited physical condition of the cervix.
When the cervix is damaged, it cannot hold the weight of the pregnancy. The cervix dilates without contractions or pain, sometimes opening completely. The dilation results in the amniotic membranes bulging through the opening and eventually rupturing, often before the baby can survive outside of the uterus. This irritates the uterus and brings on pre-term labor. In many cases, labor is detected when it is too far advanced to stop the process.
Cervical incompetence accounts for about 15 to 20 percent of all pregnancy losses during the second trimester.
Risk factors for an incompetent cervix are: a history of incompetent cervix with a previous pregnancy, surgery, cervical injury, DES (diethylstilbestrol) exposure, and anatomic abnormalities of the cervix. A prior D&C can, for example, damage the cervix.
Other causes of cervical weakness include cervical cautery (to remove growths or stop bleeding) and cone biopsy (removal of a cone-shaped section of tissue for study to detect possible precancerous growth). Prior to pregnancy or during the first trimester, there is usually no method to determine whether the cervix will eventually be incompetent.
Women with incompetent cervix typically present with "silent" cervical dilation (i.e., with minimal uterine contractions) between 16 and 28 weeks of gestation. They present with significant cervical dilation (2 cm or more) and minimal symptoms. When the cervix reaches 4 cm or more, active uterine contractions or rupture of membranes may occur.
Once the problem of incompetence is diagnosed, the condition may be treatable through a surgical procedure called cerclage (stitching the cervix closed). One or more stitches are placed around or through the cervix to keep it tightly closed.
This is usually performed after the twelfth week of pregnancy, the time after which a woman is least likely to miscarry for other reasons - but it is not done if there is rupture of the membranes or infection.
After surgery, the mother is carefully monitored to check for infection and contractions, which are sometimes brought on by the procedure. After hospital discharge, the patient may remain on bedrest in order to remove any pressure on the cervix and increase the chance of retaining the pregnancy until the baby is viable. The cerclage is usually removed just before childbirth so that the patient can give birth vaginally. In some cases, the cerclage may be left in place, and the baby is then delivered by cesarean section.