Objective The purpose of this study was to estimate the effect of bacterial vaginosis on midtrimester cervical length in women at increased risk for recurrent spontaneous preterm birth. Study Design We conducted a secondary analysis of prerandomization data from a multicenter trial of ultrasound-indicated cerclage. Women with previous spontaneous preterm birth at <34 weeks' gestation underwent initial cervical length assessment and vaginal fluid collection at 16-21 weeks 6 days gestation. Gram stains were scored with Nugent criteria. With serial scans, the shortest cervical length was observed. Results Records for 949 women had complete data. In unadjusted regression models, Nugent score (P = .003) and vaginal fluid pH (P = .008) were related inversely to cervical length. Women with bacterial vaginosis based on Nugent score <7 (P = .04) or pH <5 (P = .016) had significnatly lower cervical length than unaffected women; however, all of these effects were null after covariate adjustment. Conclusion Nugent score, pH level, and bacterial vaginosis are associated inversely with cervical length; however, these relationships become null after adjustment for relevant covariates. © 2011 Mosby, Inc. All rights reserved.