Uniform International Method to Measure Cervical Length: Are We There Yet?

Cervical length is an important predictor of spontaneous preterm birth. So far, the best way to measure cervical length has not been established. We aimed to compare the incidence of short cervical length between three methods of cervical length measurement with and without the inclusion of cervico-isthmic complex (CIC) (six methods in total) and to determine the positive predictive value for spontaneous preterm birth.

We performed a prospective single-center cohort study in women with a singleton pregnancy between August 2014 and December 2018. During the routine fetal anomaly scan (18-22 weeks), women were offered transvaginal ultrasound for cervical length measurement to screen for the risk of spontaneous preterm birth. Each cervix was measured in six different ways: single-line, two-line, and tracing method between the internal and external os of the cervix with and without CIC. We evaluated the predictive value of the different measurements for spontaneous delivery before 37 weeks using positive predictive values.

Our final study population comprised 1,691 women. The overall rate of preterm birth <37 weeks was 8.0% (4.6% spontaneous, 3.4% iatrogenic preterm birth). The mean gestational age at cervical length measurement was 19+6 weeks. The different measuring techniques resulted in significant different cervical lengths, showing a maximum difference of >8 mm between the techniques (41.04 mm [SD 7.1] with one-line without CIC and 49.18 [SD 9.05] mm with trace with CIC) with an incidence of short cervical length below <25 mm ranging from 0.4% to 1.1% (p = 0.18). The positive predictive values for spontaneous preterm birth <37 weeks ranged from 42.9% to 20.0%.

Different measurement methods for cervical length resulted in statistically significant differences in measured cervical length. Depending on the chosen cut-off this translates to different incidences of short cervical length and influences the number of women designated as high risk for preterm birth and receiving treatment. For interpretation and comparability between (inter-) national studies, it is important to adequately report on the employed technique. Future research should focus on determining the optimal measuring technique and a universal method of measurement.