Jason K. Baxter, C. David Adair, Michael J. Paidas, Matthew K. Hoffman, Frances J. Kivel, Deb Cebrik, Michael G. Ross

Preterm birth remains a major cause of neonatal morbidity and mortality worldwide. Short cervical length as measured by transvaginal ultrasound in the second trimester represents the single most predictive risk factor for spontaneous preterm birth. Previous studies have addressed, in part, the limitations of transvaginal ultrasound availability by utilizing a cervicometer to screen patients for short cervix, identifying those patients who may not benefit from transvaginal ultrasound cervical length screening. In view of the prior studies indicating that a cervicometer measurement may have a high negative predictive value for a sonographically short cervix, we sought to identify the ideal cervicometer threshold value in a prospective, multicenter study. The primary objective was to determine the cervicometer cervical length measurement threshold that provides a high negative predictive value for the identification of patients who are highly unlikely to have a transvaginal ultrasound cervical length measurement <20 and <25 mm and, therefore, may forego transvaginal ultrasound. This prospective study, executed in 5 US centers, included 401 women ≥ 18 years of age who provided written informed consent to undergo cervical length measurement in the mid trimester. All women underwent both cervicometer and transvaginal ultrasound measured cervical lengths by individuals blinded to results of the other measurement. Both measurements were performed at 17-23 weeks' gestation (Visit 1) and repeated at 24-29 weeks' gestation (Visit 2). All transvaginal ultrasound measurement images were reviewed by a central reader. Test characteristics and receiver operating characteristic curves were created to determine and confirm the optimal cervicometer cervical length threshold, maximizing the negative predictive value. Three hundred fifty eight subjects were evaluable at Visit 1 and 267 at Visit 2. At Visit 1, the average transvaginal ultrasound cervical length was 38.7± 7.6 mm and the average cervicometer cervical length was 30.3 ± 8.8 mm. Similar measurements were seen at Visit 2. Receiver operating characteristic curves were utilized to graphically identify a cervicometer cervical length threshold of 30 mm that maximized sensitivity while minimizing the false positive rate. The 30 mm cervicometer cervical length threshold provided a 98-100% negative predictive value and 0.0 negative likelihood ratio for identification of women who have a low likelihood to have a sonographic short cervix (i.e., transvaginal cervical length <20 mm or <25 mm). The 17-23 weeks' gestation 30 mm cervicometer cervical length threshold has 100% sensitivity, 45-46% specificity, and 1.8 and 0.0 positive and negative likelihood ratios to predict sonographic cervical length ≤20 and ≤25 mm. Cervicometer cervical length screening successfully identifies women at low risk for short transvaginal cervical length. Use of a 30 mm threshold by cervicometer cervical length measurement confers a 98-100% negative predictive value, with high sensitivity and moderate specificity to predict a transvaginal ultrasound short cervical length. Cervicometer measurement of cervical length may permit almost 50% of women to avoid transvaginal ultrasound.