Which part of cervical length is predictive of preterm birth in women with cerclage?

To evaluate the predictive value of transvaginal ultrasonography measures: total cervical length (TCL), length to internal OS (LIO) and external OS (LEO) from the cerclage following cerclage placement, for spontaneous preterm birth (PTB).

A retrospective cohort at a single tertiary care center (2010-2020). Women with McDonald cerclage were evaluated during the 2nd and 3rd trimesters for cervical measurements: TCL, LIO, LEO. The primary outcome was PTB < 37 weeks. Demographic data, obstetric history, delivery information and serial cervical length measurements were collected. Subgroup analyses were performed to evaluate cervical length parameters (as described) and PTB rates prior to 34, 32 and 28 weeks of gestation.

Of 66 women enrolled, 36.4% (n = 24) had PTB. There were no differences in the obstetrical history and demographic information (p > .05) nor indication for cerclage (p = .369). Cervical length measurements at 20-24 weeks demonstrated a shorter TCL & LIO (22.69 vs. 33.86 mm, p = .001; 9.25 vs. 15.9 mm, p = .0042; respectively) in the preterm group. The LEO was similar in both groups. This pattern was also shown in a subgroup analysis (PTB < 34, 32 and 28 weeks).

Cervical length from internal OS to cerclage and total cervical length have a predictive value for PTB in women with a cervical cerclage, regardless of the indication for cerclage/previous pregnancy outcomes. Our data emphasize the importance of follow-up cervical length measurements of women with a cervical cerclage, and especially cervical length from internal OS to cerclage as those women may need additional care.