Vaginal pH versus cervical length in the mid-trimester as screening predictors of preterm labor in a low-risk population.

Objective: To assess the accuracy of a sign of bacterial vaginosis and a sign of cervical insufficiency in predicting preterm labor (PTL, occurring in the 37th week or earlier) and early PTL (occurring in the 34th week or earlier) in a low-risk cohort of 316 pregnant women.

Method: Vaginal pH was assessed using test gloves and cervical length (CL) was measured by transvaginal ultrasound. A pH value less than 5.0 (the 95th-percentile threshold) and a CL greater than 26 mm (the 5th-percentile threshold) were considered normal.

Results: There were 14 participants (4.4%) with an elevated vaginal pH and 15 (4.7%) with a shortened CL. The incidence of PTL (<37 weeks) was 7.2%, while incidence of early PTL (≤34 weeks) was 2.5%. A shortened CL was significantly correlated with PTL (likelihood ratio [LR] weighted by prevalence; 2.7; 95% CI, 1.1-6.7) but not with early PTL (LR, 0.8; 95% CI, 0.4-1.8). An elevated vaginal pH was a better predictor of PTL (LR, 3.7; 95% CI, 1.3-10.4) and early PTL (LR, 1.7; 95% CI, 1.1-3.1).

Conclusion: An elevated vaginal pH was a better predictor of early PTL than a shortened CL in this cohort of pregnant women at low risk.