OBJETIVOS: avaliar a utilidade do comprimento cervical ultra-sonográfico para a predição do parto em gestantes com contrações uterinas persistentes e bolsa íntegra. MÉTODOS: realizou-se estudo observacional, prospectivo, do tipo coorte no qual foram incluídas 45 gestantes internadas em nossa instituição entre a 22ª e a 34ª semana. Ultra-sonografia transvaginal do colo foi realizada uma única vez, após completado um ciclo de tocólise endovenosa, com aferição do comprimento, conforme critérios descritos previamente. Os achados ultra-sonográficos não foram utilizados para o diagnóstico ou conduta obstétrica. O desfecho estudado foi a ocorrência de parto pré-termo (PURPOSE: to assess the role of ultrasonographic cervical length in predicting premature labor in patients presenting persistent uterine contractions and intact membranes. METHODS: a prospective observational cohort study was performed in 45 women admitted to our hospital between 22 and 34 weeks of gestation. Transvaginal sonographic evaluation of the cervix was performed once in the women who had completed a course of parenteral tocolysis. The cervical length was obtained according to criteria reported previously. Cervical sonographic findings were not used in diagnosis and management. Outcome variable was the occurrence of preterm delivery (<37 weeks). Statistical analysis consisted of univariate method with the purpose of determining the significant contribution of cervical length to the prediction of preterm delivery. The adopted significance level was 5% (p<0,05) and the confidence interval was 95% (95% CI). RESULTS: the incidence of preterm delivery was 51.11% (23/45). Cervical length was significantly associated with the outcome (p<0.0001). Receiver operating characteristic curve analysis showed that a cervical length of 20 mm was the best cutoff in predicting preterm delivery (sensitivity 86.9%; specificity 81.8%; positive predictive value 83.3%; negative predictive value 85.7%). The calculated area under the curve was 0.91 (95% CI: 0.79-0.97; p<0.0001). CONCLUSIONS: among women with persistent uterine contractions and intact membranes treated for preterm labor, a cervical length of less than 20 mm demonstrated a high likelihood of preterm birth. Transvaginal ultrasound may improve the accuracy of diagnosis in symptomatic women.