ObjectiveTo improve the accuracy of the diagnosis of preterm labor by comparing transvaginal sonography to digital examination of the cervix.MethodsWe performed transvaginal sonography in women with preterm labor who had completed a course of parenteral tocolysis. Cervical length was measured according to criteria reported previously. Cervical sonographic findings were not used in diagnosis or management. Sonographic cervical length was compared to digital assessment of dilation and effacement to assess the risk of preterm birth after treatment for preterm labor.ResultsForty-eight singleton and 12 twin gestations were studied. Thirty women were nulliparous and 30 were parous. The mean (± standard deviation) gestational age was 31.1 ± 2.7 weeks (range 24–35) at the examination and 35.6 ± 2.9 weeks (range 26–43) at delivery. Twenty-four subjects delivered before 36 weeks' gestation and 36 delivered at or after 36 weeks. Cervical sonography was distinctly superior to digital assessment of dilation and effacement as a test for delivery before 36 weeks, when compared using receiver operating characteristic curves. This analysis indicated a cervical length of 30 mm as the best cutoff to maximize sensitivity and specificity. All 24 subjects who delivered preterm had cervical lengths less than 30 mm. Cervical sonography was especially useful in selecting women with preterm labor who would not deliver prematurely, ie, a high negative predictive value. None of 15 women whose cervical length was 30 mm or more delivered spontaneously before 36 weeks.ConclusionsAmong women treated for preterm labor, a cervical length of at least 30 mm predicted a low likelihood of preterm birth. Cervical sonography may improve the accuracy of diagnosis in women treated for preterm labor.