Reproducibility of Cervical Length Measurement Throughout Pregnancy.

Objectives: To assess the intra- and interobserver variability of cervical length (CL) measurement in the three trimesters of pregnancy. Methods: Prospective observational study. Transvaginal CL measurement was performed by two operators ignorant of each other's results. Two methods of measurement were assessed: the straight line method (SL = from internal to external cervical os along the endocervical mucosa) and the two-line method (TL = the sum of two lines following the curvature of the endocervical canal). Results: There were 236 sets of CL measurements of which 96 (40.7%), 73 (30.5%), and 67 (28.8%) were performed in the first, second, and third trimesters of pregnancy, respectively. Intra- and interobserver variability was excellent for both methods (ICC 0.90) and was not influenced by gestational age. Mean difference was 1 mm (LOA -4 to -3 mm) for the same examiner and 1 mm as well between examiners (LOA -5 to -7 mm). Mean CL was slightly longer for the 2-L method (33 mm, SD = 5.2) compared to the SL method (31.4 mm, SD = 4.9). There was significantly more bias in the difference between methods the longer the CL measurement was (p <.001). Conclusions: CL measurement shows excellent intra- and interobserver variability across gestation and the variability of the measurement is not influenced by gestational age or CL length for either method. The TL method produces slightly longer values.