What causes cervical incompetence (weakness)?
The known causes of cervical incompetence include previous therapeutic dilatation of the cervix (D&C), which is done in termination of pregnancy.
Cone biopsy - a mode of diagnosis and treatment for a severely abnormal smear - is another.
Both these procedures may subsequently lead to cervical incompetence but this is quite uncommon. Most gynaecologists will avoid forcibly dilating the cervix whenever possible, especially since alternatives are available, in many cases.
What happens in a miscarriage due to cervical incompetence?
In the majority of cases, it is without warning. There is a gush of fluid as the membranes of the gestational sac break. This follows a silent opening of the weakened cervix.
Once the "waters" have broken, the process is virtually irreversible. The woman will proceed to miscarry in a matter of hours.
If a diagnosis of an incompetent cervix is made, following such a miscarriage, remedial action has to wait until she has conceived again. Corrective action is taken after fourteen weeks, when the possibility of spontaneous miscarriage from other causes has receded to negligible, and a scan has verified that the fetus and placenta are growing normally. Fourteen weeks is also just before entering the danger period of miscarriage caused by cervical incompetence.
What does corrective action involve?
The commonly used method is to put a special suture in the weakened cervix to close it and hold it in that closed state. The procedure is called "cervical cerclage" and, depending on how the suture is made, names for the procedure such as Shirodikar and Macdonald will be used. It is in most cases done vaginally, and the suture is subsequently removed at thirty-eight weeks of gestation, ready for labour.
The insertion of the stitch is done in theatre under a general anaesthetic. Many obstetricians will advise a subsequent hospital stay of at least a day, for complete rest and observation. There is a small risk that the action of putting the suture in could trigger a miscarriage, hence the precaution. Removal of the suture is straightforward and does not require going to theatre or an anaesthetic.
The success rate of vaginal route cervical cerclage is modest but actual figures are not clearly known. It is now being more and more accepted that probably the more difficult procedure of putting the stitch in abdominally rather than vaginally may be the better method, with a better success rate. With the abdominal method, the suture is applied to the cervix through the abdomen, hence it is a bigger and technically more difficult operation. It, therefore, may not be available in all hospital units.