Background: Preterm birth is a major cause of death and a significant cause of long-term loss of human potential amongst survivors all around the world. Complications of preterm birth are the single largest direct cause of neonatal deaths. Aim of the work: is to evaluate and compare the diagnostic value of vaginal pH and cervical length measurement in the second trimester of pregnancy as a preterm labor predictor. Patients and methods: This prospective analytical study was done on a cohort of 100 selected pregnant women who were fulfilled the inclusion criteria in their antenatal visits at Al-Azhar university hospital. Patients coming for routine antenatal care between 18-and 24-weeks gestations were enrolled in the study. Results: The results showed that the minimum cervical length measured was 2.1 (Cm) and the maximum was 4.4 (Cm). The mean cervical length was 3.71 ± 0.64(Cm). The minimum vaginal pH was 3.5 and the maximum pH was 5.5. The mean vaginal pH was 4.7 ± 0.6. The statistical analysis found that there is a significant correlation between cervical length and preterm labor. Also, cervical length had better predictive parameters than vaginal pH. Conclusion: This concludes that measuring cervical length by trans vaginal ultrasound has a better predictive value for preterm labor than measuring vaginal pH in the second trimester of pregnancy.