Cervical incompetence is an important, but undoubtedly over-diagnosed, condition. In 1658, Grant gave quite an elegant description:
...the orifice of the womb is so slack that it cannot rightly contract itself to keep in the seed; which is chiefly caused by abortion or hard labour and childbirth, whereby the fibres of the womb are broken in pieces from one another and they, and the inner orifice of the womb overmuch slackened.
A diagnosis of cervical incompetence is usually made on the basis of a woman's past pregnancy history. Classically this is following one or more late second trimester or early third trimeser losses (stillbirth). Usually they begin with painless leaking of liquor (the waters), or finding during pregnancy a gradual painless dilatation of the cervix, with membranes bulging into the vagina.
What Causes CI?
An incompetent cervix is thought to be due to either an inherent (congenital) weakness of the cervix tissues or a result of forced dilatation or trauma. Congenital weakness can occur unexpectedly or as a result of exposure to DES (diethylstilbestrol) before birth.
Studies have found that if surgical dilatation of the cervix is performed, the risk of CI depends upon the number and degree of dilatation used. It is unlikely to occur because of a diagnostic D&C (e.g. for irregular periods) or after a miscarriage, when the cervix is already starting to open. No increase in risk with up to two first trimester surgical terminations of pregnancy has been found. Three or more does carry an increased risk, of about 12%.
There is no clear-cut way of determining if a particular surgical event has caused it, either. Taking of biopsies as part of investigating an abnormal smear (Pap) test does not increase the risk. A less commonly performed procedure now is called Cold Knife Cone biopsy. This involves a general anaesthesia and removal of a wedge of the cervix to treat cervical pre-cancer This is associated with about a 1 in 50 (2%) risk of CI. The more commonly performed Loop diathermy or Large Loop Excision of the Transformation Zone (LLETZ) has not been found to be associated with an increased risk.
What Can be Done About It?
Cervical cerclage is the treatment that is offered. This involves placing a stitch high up around the cervix to try & keep it closed. The stitch can be placed either from down below (vaginally) or via an abdominal incision. The latter is usually used when vaginally placed stitches fail. They are called McDonald or Shirodkar stitches. The Shirodkar variant involves a bit more extensive surgery to ensure the stitch is high up on the cervix.
The stitch is usually removed around 37 weeks and labour ensues fairly rapidly if the diagnosis was correct. Abdominal cerclage requires an elective caesarean section and the stitch is usually left in-situ for future pregnancies.
Complications of the stitch include rupture of the membranes at the time of placement, and increased risk of infection.
Does it Improve Pregnancy Outcome?
There have been a couple of proper randomised controlled studies to try and find the answer to this question. The largest was the UK MRC/RCOG trial, published in 1993, where 1292 women were randomised to cerclage or control. The study did find an improvement in outcome in women who had experienced a previous second trimester loss and had a cerclage placed. There were more deliveries in the treated group after 33 and 37 weeks. The reduction in premature delivery rate was equivalent to an improved outcome for 1 of every 25 women who had a stitch placed. So, although the benefit was shown, it certainly did not work for everyone.
The important thing about this study is that it did include women thought to be at risk because of previous surgery, terminations, etc., but the only group that showed benefit was those who had a previous pregnancy loss. This is the major challenge in this area - detection of those at risk of pregnancy loss/early delivery before it happens.
How Can Women with Suspected CI be Detected?
There are descriptions of cervical assessment before pregnancy to try and detect those who may benefit from a stitch. These include checking cervical resistance or compliance, with a dilator, or specialised instrument. Whilst promising, no studies have yet found a predictor of poor outcome as good as a previous pregnancy loss.
Transvaginal ultrasound (TVS) during pregnancy has shown some promise. The usual length of the cervix is about 4cm as measured on TVS. Women with a cervical length of less than 2.5cm have been found to have a 50% risk of preterm delivery in one study. Other studies have looked at opening of the internal section of the cervix ('funnelling' or 'beaking') in response to pressure on the top of the uterus. It does seem that this finding early in pregnancy is suggestive of cervical incompetence and that the findings are progressive throughout pregnancy.
These studies are really still at an early stage and it takes a great leap to presume that on the basis of these findings alone a stitch will improve things. Larger observational studies and a cerclage study on the scale of the MRC/RCOG will hopefully follow and define the place of this investigation. At the present, this type of scan is generally confined to research centres in the UK and is certainly not to be considered 'standard of care'.