Background: Shortened mid-trimester cervical length in the mid-trimester period can predict very early spontaneous preterm birth. On the other hand, bacterial vaginosis, as a risk factor of the preterm labor, can increase the vaginal pH. This study aimed to evaluate the effect of cervical length and vaginal pH on the prediction of preterm labor in low risk population during the mid-trimester to prevent the preterm labor using the appropriate interventions.

Materials and Methods: This cohort study was performed on 438 pregnant women between 18 and 24 weeks of pregnancy. The vaginal pH, cervical length and delivery gestational age were determined. The risk of preterm labor was evaluated based on pH and the cervical length and then the predictive values of them were determined.

Results: The odds ratio of preterm labor in alkaline toward acidic vaginal pH was more than 3 times (OR=3.06). Moreover, a significant relationship was seen between the cervical length and preterm labor. The chance of preterm labor in women with a cervical length less than 30mm was increased 14 times compared to those with a normal cervical length. More than 71% of the women had preterm labor.

Conclusion: The risk of early preterm labor in alkaline vaginal pH is higher than the late preterm and the risk of late preterm labor in short cervix is higher than the early preterm.