Objective: The study's objective was to determine the correlation and agreement between transperineal ultrasonography and transvaginal ultrasonography in the assessment of cervical length in gravid patients.
Study design: After a pilot, unblinded series of transperineal and transvaginal cervical length measurements in 200 gravid patients, 206 study patients between 14 and 34 weeks' gestation with intact membranes and cervical dilatation of < or = 2 cm underwent transperineal and transvaginal cervical length assessment under a blinded, 2-sonographer protocol. The Pearson correlation coefficient, Lin concordance coefficient, and Bland-Altman plot were used. Acceptable concordance was defined as > 0.82, with an acceptable correlation of > 0.9 and an acceptable difference between the means of < 3 mm. The power of the study to detect this degree of concordance was estimated to be 95% at this sample size.
Results: Paired ultrasonographic measurements were obtained for all 206 study patients. Transperineal mean cervical length was 35 +/- 8.6 mm. Transvaginal mean cervical length was 35.9 +/- 8.8 mm. The Pearson correlation coefficient was 0.959, and the Lin concordance coefficient was 0.955, with a 95% confidence lower bound of 0.949. Close agreement between transperineal and transvaginal measurements was observed across the full range of cervical lengths (1-5 cm). The estimated difference between the paired means was 1 mm. The 95% tolerance interval for any given paired observation (Transperineal length - Transvaginal length) was -5.7 to +4 mm.
Conclusions: Cervical length measured by transperineal ultrasonography demonstrates close correlation and agreement with transvaginal measurements. With sonographer experience and optimal technique, approximately 95% of transperineal cervical length observations can be expected to be within 5 mm of a given paired transvaginal measurement. Transperineal ultrasonography may be a preferred method of cervical length assessment for situations in which vaginal placement of instruments should be minimized.