Are a short cervix and a history of preterm birth absolute indications for cervical cerclage?

• 1) a history of second-trimester loss involving painless cervical dilatation in the absence of infection, bleeding, amniorrhexis, and fetal demise
  
• 2) asymptomatic cervical changes in the current pregnancy.
  

Although our understanding of cervical insufficiency has undergone many revisions and reinterpretations in the intervening years, we still lack an accepted diagnostic test or proven criteria for diagnosis. Cerclage is placed in 1% of all pregnancies in the United States, but there is no consensus on indications, and the effectiveness is still a matter of debate.¹ Cerclage placement based on ultrasonographic (US) measurement of cervical length has been proposed as the solution to this clinical quagmire, in the wake of evidence suggesting that cervical length may act as a surrogate for cervical competence.

A patient-level meta-analysis of four randomized trials of cervical cerclage, published in 2005, reconfirmed the original indication for cerclage. In women who had a cervical length below 25 mm, cerclage reduced the rate of preterm birth at less than 35 weeks’ gestation only if they had a history of preterm birth.² This finding prompted the question: Would such women represent a truly homogeneous population in terms of therapeutic response to cerclage?

The Owen trial attempts to answer this specific question.

Details of the trial

Women who had a cervix shorter than 25 mm and a history of preterm birth and who were pregnant with a singleton gestation were eligible. Candidates for elective cerclage based on history, or for emergency cerclage based on cervical dilatation of at least 2 cm with visible membranes, were excluded from the study—possibly reducing the generalizability of the findings.

For the remaining 302 participants, cerclage appeared to have an overall benefit when survival analysis took the duration of gestation into consideration. But only women who had a cervical length below 15 mm had a significant reduction in the primary outcome (preterm birth at less than 35 weeks’ gestation) with cerclage. These results are somewhat reminiscent of the findings of a randomized comparison of cerclage and 17α-hydroxyprogesterone caproate in women who had a short cervix (measured by US), in which cerclage proved to be superior only when cervical length was less than 15 mm.³

Why the 15-mm cutoff isn’t definitive

Despite these findings, the 15-mm measurement cannot be assumed to be completely prescriptive because it was selected somewhat arbitrarily. Furthermore, it may be inadvisable to wait for cervical length to decrease below 15 mm. In pregnancies in which cervical length is below 5 mm, there is a significantly higher expression of intra-amniotic inflammation than in those in which cervical length is 6 to 25 mm, according to another recent study.⁴ Women who have a very short cervix may be far along the inflammatory cascade and may have already entered the irreversible phase of parturition, reducing the efficacy and even the advisability of cerclage. A positive fetal fibronectin test (as a marker of inflammation and choriodecidual disruption) and an increased level of interleukin-8 in cervical mucus reportedly identified a subgroup of women with a short cervix who would not benefit from cerclage—and who might even be harmed by it.^5,6

Because preterm birth is such a complex disorder, it is unlikely that one intervention will be effective in all women—even within a certain stratum of cervical length. Rather, it may be necessary to identify subsets of pregnant women amenable to targeted or tailored intervention.