Ultrasonography for cervical length measurement: agreement between transvaginal and translabial techniques

Abstract

Objective: To assess agreement between transvaginal and translabial ultrasonography for measuring cervical length in the second and third trimesters. Methods: Eighty-four obstetric patients at 14–40 weeks’ gestation were enrolled. Three images of the cervix were obtained by translabial (3.5-MHz curvilinear transducer) and transvaginal (6.5-MHz endovaginal probe) ultrasonography. The mean cervical length obtained by using each method was used for data analysis. The McNemar χ 2 test was used to assess the difference between techniques in their ability to obtain a measurement. The paired t-test was used to evaluate the differences between the measurements. The mean difference and SD for the differences were used to calculate the limits of agreement. An acceptable difference was defined as less than 0.5 cm. Results: Cervical length measurements were obtained in 84 patients (100%) by using the transvaginal technique and in 80 patients (95%) by using the translabial technique ( P = .1). Eighty patients had both transvaginal and translabial measurements for comparison. The mean difference in cervical length was 0.37 cm ( P < .001; 95% confidence interval [CI] 0.21, 0.52). The upper and lower limits of agreement were 1.75 cm (95% CI 1.48, 2.02) and −1.01 cm (95% CI −0.74, −1.28), respectively. The differences between the two measurements were within these limits 95% of the time. These limits of agreement were greater than the acceptable difference of 0.5 cm. Conclusion: Transvaginal and translabial techniques should not be used interchangeably for clinical assessment of cervical length because agreement between the methods is not within an acceptable range.