Cervical length at 11-40 weeks: unconditional and conditional longitudinal reference ranges.

INTRODUCTION: Our aim was to establish unconditional and conditional longitudinal reference ranges for cervical length throughout pregnancy.

MATERIAL AND METHODS: Prospective longitudinal study. In singleton pregnancies repeated cervical length measurements were carried out by transvaginal ultrasound throughout gestation. Multilevel modeling was applied to establish cervical length reference ranges from 11 to 40 weeks.

RESULTS: In all, 4397 women contributed to 13 765 cervical length measurements. A linear mixed effects random intercept-random slope model was fitted to the data. Mean cervical length had a negative non-linear polynomial association with gestational age. Unconditional ranges were developed. Terms that allow the construction of personalized cervical length charts conditional to a previous measurement were calculated.

CONCLUSIONS: We constructed longitudinal reference charts for cervical length in singleton pregnancies. Cervical length should be adjusted according to specific gestational-age-dependent ranges. Individualization of cervical assessment is feasible by the application of charts conditional to previous measurements.