Comparison of three methods of cervical measurement in the first trimester: single-line, two-line, and tracing

To compare three methods of cervical length measurements using ultrasound in the first trimester: single-line, two-line, and tracing.

Images appropriate for cervical length measurements were obtained using transvaginal ultrasound between 11 + 0 and 13 + 6 weeks' gestation. The cervix was measured on stored images using three methods: a single straight line, two segments (two-line method), and by tracing the distance along the cervical canal. Results were compared for intraoperator repeatability and interoperator variability. The degree of the cervical curvature was measured.

The mean cervical measurement using the single-line measurement was 32.8 (SD 4.4) mm. The mean cervical length was essentially the same using the two-line and tracing measurement: 34.4 (SD 4.7) mm and 34.5 (SD 4.7) mm, respectively. Both intra- and interoperator agreement was high. The degree of curvature of the cervical canal was variable, with a resultant variability in the difference between the single straight line measurement and the other two types of measurement.

Using stored images, each of the measurement techniques is highly reproducible. Both the two-line and the tracing methods yield larger measurements than the single-line technique. Therefore, it appears that in select cases the straight line measurement significantly underestimates the actual cervical length.