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

Objective: To assess agreement between transvaginal and translabial ultraso nography for measuring cervical length in the second and third trimesters. Methods: Eighty-four obstetric patients at 14-40 weeks' gestation were enro lled. Three images of the cenix were obtained by translabial (3.5-MHz curvi linear transducer) and transvaginal (6.5-MHz endovaginal probe) ultrasonogr aphy. The mean cervical length obtained by using each method was used for d ata analysis. The McNemar chi(2) test was used to assess the difference bet ween techniques in their ability to obtain a measurement. The paired t-test was used to evaluate the differences between the measurements. The mean di fference and SD for the differences were used to calculate the limits of ag reement. An acceptable difference was defined as less than 0.5 cm. Results: Cervical length measurements were obtained in 84 patients (100%) b y using the transvaginal technique and in 80 patients (95%) by using the tr anslabial technique (P = .1). Eighty patients had both transvaginal and tra nslabial measurements for comparison. The mean difference in cervical lengt h was 0.37 cm (P < .001; 95% confidence interval [CI] 0.21, 0.52). The uppe r 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 mea surements were within these limits 95% of the time. These limits of agreeme nt were greater than the acceptable difference of 0.5 cm. Conclusion: Transvaginal and translabial techniques should not be used inte rchangeably for clinical assessment of cervical length because agreement be tween the methods is not within an acceptable range. (Obstet Gynecol 2000;9 6: 554-8. (C) 2000 by The American College of Obstetricians and Gynecologis ts.).