The cervix, which has the duty of keeping your womb closed during pregnancy, is put under increasing pressure as your baby grows and presses on it. It is not supposed to open (dilate) until you go into labor. If you have an incompetent cervix, the additional pressure makes it open too early. This can ultimately result in a miscarriage (if it happens earlier in pregnancy), or pre-term labor (if it happens later on). An incompetent cervix can be caused by a variety of things, including malformations of the uterus or cervix, previous cervical surgery or a curettage, or if you were exposed to the drug DES in your mother's womb.
Having an incompetent cervix is relatively rare and it affects only one or two percent of all pregnancies. At the same time, many second trimester miscarriages are caused by this condition. An incompetent cervix is diagnosed through an abdominal ultrasound, or a pelvic exam. But these examinations are not routine, and usually only carried out if you have risk factors or previous miscarriages. The treatment is a cerclage, a surgical intervention that keeps your cervix shut until you give birth. Cerclages are usually placed earlier on in pregnancy, at the beginning of the second trimester, and they are very effective at giving your cervix the help it needs to remain closed.