The purpose of this study was to assess whether vaginal sialidases level in early pregnancy is associated with preterm birth among women who are bacterial vaginosis-positive. Of the 1806 women who were enrolled at < 20 weeks of gestation, 800 of the women were bacterial vaginosis-positive (Nugent score, 7-10); 707 of the women had birth outcome data; 109 of the women who were bacterial vaginosis-positive had an adverse preterm outcome, which included 53 spontaneous preterm births (19 births were early at 20-34 weeks, and 34 births were late at 34-37 weeks), and 14 of the women had late miscarriages (12-20 weeks). Sialidase levels were compared with 352 control subjects (term normal birthweight infants). Sialidase levels at ≥ 5, ≥ 10, and ≥ 14 nmol (odds ratio [OR], 1.56; 95% confidence interval [CI], 1.01-2.41; OR, 2.14; 95% CI, 1.25-3.64; OR, 3.17; 95% CI, 1.64-6.10, respectively) was associated significantly with all adverse preterm outcomes. The ≥ 10 nmol and ≥ 14 nmol cut-points were associated strongly with early spontaneous preterm births (OR, 3.79; 95% CI, 1.42-10.10 and OR, 5.36; 95% CI, 1.77-16.23, respectively) and late miscarriages (OR, 4.87; 95% CI, 1.61-14.65; OR, 8.33; 95% CI, 2.57-26.9, respectively). Elevated sialidase level that is measured at 12 weeks of gestation is associated strongly with early spontaneous preterm births and late miscarriage. Copyright © 2011 Mosby, Inc. All rights reserved.