Background and Objective: Early prediction of prolonged pregnancy for prevention of fetal asphyxia is important. The present study aimed to examine transvaginal ultrasonographic measurement of the cervical length at week 37 as a predictor of prolonged pregnancy. Materials and Methods: This study was a descriptive-correlated design of ultrasound examination at week 37 of gestation in 126 singleton pregnant women with cephalic presentation attending for prenatal care at the southern Tehran health antepartum clinics. The length of the cervix was measured using transvaginal ultrasound and clinical and demographic data questionnaire. Gestational age at the time of delivery was calculated. Prolonged pregnancy was defined as a pregnancy that extended beyond 41 weeks of gestation. T-test, χ2 test, Logistic Regression and receiver operating characteristic curve was used for statistical analysis using the SPSS ver. 16.0. Results: The mean cervical length was 26.10 ±8.9. The rate of pregnancy that extended beyond 41 weeks of gestation in this study was 4.8%. The cervical length at week 37 was significantly longer in women who delivered at >41 weeks (n=6) than in those delivering at ≤41 weeks (n=120), (34.08 mm ±8.5 vs. 25.7 ±8.7 mm P=0.02). The best cut-off value of cervical length at week 37 for the prediction of prolonged pregnancy was 25.5 mm, with a sensitivity of 100%, specificity of 50%, positive predictive value of 9%, and negative predictive value of 100%. Conclusion: Cervical length at week 37 can predict the likelihood of prolonged pregnancy, but with low positive predictive value. This information can be utilized when counseling patients regarding the management of term pregnancies.