OBJECTIVETo estimate whether antibiotic treatment of asymptomatic women with a positive cervical or vaginal fetal fibronectin test in the second trimester would reduce the risk of spontaneous preterm delivery.METHODSWomen were screened between 21 weeks 0 days and 25 weeks 6 days of gestation with cervical or vaginal swabs for fetal fibronectin. Women with a positive test (50 ng/mL or more) were randomized to receive metronidazole (250 mg orally three times per day) and erythromycin (250 mg orally four times per day) or identical placebo pills for 10 days. The primary outcome was spontaneous delivery before 37 weeks' gestation after preterm labor or premature membrane rupture.RESULTSA total of 16,317 women were screened for fetal fibronectin, and 6.6% had a positive test; 715 fetal fibronectin test–positive women consented to randomization. Outcome data were available for 703 women: 347 in the antibiotic group and 356 in the placebo group. The antibiotic and placebo groups were not significantly different for maternal age (P= .051), ethnicity (P= .849), marital status (P= .127), education (P= .244), and bacterial vaginosis (P= .236). No difference was observed in spontaneous preterm birth before 37 weeks' (odds ratio [OR] 1.17, 95% confidence interval [CI] 0.80, 1.70), less than 35 weeks' (OR 0.92, 95% CI 0.54, 1.56), or less than 32 weeks' (OR 1.94, 95% CI 0.83, 4.52) gestation in antibiotic-compared with placebo-treated women. Among women with a prior spontaneous preterm delivery, the rate of repeat spontaneous preterm delivery at less than 37 weeks' gestation was significantly higher in the active drug compared with the placebo group (46.7% versus 23.9%,P= .039).CONCLUSIONTreatment with metronidazole plus erythromycin of asymptomatic women with a positive cervical or vaginal fetal fibronectin test in the late second trimester does not decrease the incidence of spontaneous preterm delivery.