Reproducibility of cervical length measurement throughout pregnancy.

To assess the intra- and interobserver variability of cervical length (CL) measurement in the three trimesters of pregnancy. 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). 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 ( 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.