Cervical cerclage

As discussed earlier, cervical competence is not a discreet entity but should be considered to be a continuum. Abnormalities of cervical function may be a major factor or a minor contributor to the biochemical and mechanical events that lead to preterm delivery. There is probably considerable overlap between the mechanisms of second trimester pregnancy loss and early preterm delivery. It is clear that in women whose history strongly suggests cervical weakness, for example, those with a past history of cervical surgery or those with recurrent episodes of rapid relatively painless second trimester fetal loss, cervical cerclage will significantly improve the prospects for success in subsequent pregnancies. Where the aetiology of previous second trimester pregnancy losses or preterm deliveries points less clearly to an obvious role for cervical weakness, then whether to insert a cervical cerclage is largely a matter for individual clinical judgement. A short cervix prior to pregnancy or in early pregnancy, relatively rapid or painless early preterm deliveries, an absence of symptoms of dysmenorrhoea, all point to the possibility that cervical dysfunction may contribute to preterm delivery. An association between preterm delivery and chorionamnionitis does not necessarily discount a cervical problem since, as discussed earlier, there is an interplay between cervical function and genital tract microbiology which means that, even in cases where cervical function is undoubtedly abnormal, there is likely to be a degree of chorionamnionitis associated with preterm delivery. Preterm deliveries which are beyond 32 weeks or which are associated with major placental abruption, fetal growth restriction or pre-eclampsia are less likely to have a cervical element in their aetiology.

There have been few studies of the benefit of cervical cerclage in reducing the risk of preterm delivery partly because the views of obstetricians in this area are polarized and it has been difficult to persuade clinicians to randomize their patients into trials. The Royal College of Obstetricians and Gynaecologists/Medical Research Council (RCOG/MRC) trial showed that cervical cerclage does reduce the risk of preterm delivery but that 25 patients would need to receive a cerclage for it to benefit one patient. Although it was previously widely believed that cervical cerclage increased the risk of genital tract infection, there is not good evidence for this. Nevertheless there are clearly risks associated with the actual insertion of cervical cerclage and there has, therefore, been interest in trying to more precisely target cervical cerclage. There have been several studies in which women previously defined as at high risk of preterm delivery have had serial ultrasound measurements of cervical length performed with cerclage being performed when cervical length reaches a predetermined cut off. The Cipract study randomized women found to have a cervical length of 25 mm or less before 27 weeks to either cervical cerclage and bed rest or bed rest alone. This study showed a significant benefit of cerclage in reducing the preterm delivery rate and improving neonatal morbidity. Rust et al. (2005) randomly assigned 138 women whose cervical length was less than 25 mm between 16 and 24 weeks to cerclage or no cerclage and showed no benefit of cerclage. However, in this study there was a delay in the introduction of cerclage to allow the results of amniocentesis to be obtained and a higher incidence of placental abruption.

More recently To et al. (2004) randomized 255 women from a low-risk population whose cervical length was found to be 15 mm or less at a single ultrasound examination at 22-24 weeks to either cerclage or no cerclage and found that although strategy identified a group of women who were at high risk of early preterm birth, cervical cerclage did not reduce that risk. The screening event in this study was, however, relatively late in pregnancy. The study therefore inevitably excluded any women having a late second trimester pregnancy loss or very early preterm delivery and the failure of cervical cerclage to be beneficial may have been due to the fact that those women who had the potential to benefit from cerclage had already developed biochemical and mechanical changes in the lower pole of the uterus which made their preterm delivery inevitable.

If ultrasound indicated cervical cerclage is to be used the appropriate threshold has not yet been established. Groom et al. have shown that the presence of visible fetal membranes at the time of cervical cerclage is a strong prognostic indicator for the risk of preterm delivery. Visible fetal membranes are never seen at a cervical length greater than 15 mm. The threshold for cervical cerclage should therefore probably be greater than 15 mm which may also explain the lack of positive findings in the large study of To et al. Given the fact that the data on ultrasound indicated cervical cerclage is currently limited and variable in its conclusions further evaluation of this strategy is required before it is used widely in routine clinical practice.

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