METHODS: A multicenter, double-blind, randomized, placebo controlled noninferiority trial was conducted in antenatal clinics in Thailand, the Philippines, Vietnam, and Argentina. Pregnant women seeking antenatal care between March 2004 and March 2007 who met the inclusion and exclusion criteria were invited to participate in the study. Those who consented were randomly allocated to receive either a 1-day or a 7-day course of 100 mg capsules of nitrofurantoin, which was taken twice daily. The primary outcome was bacteriologic cure on day 14 of treatment.RESULTS: A total of 1,248 of 24,430 eligible women had asymptomatic bacteriuria, making the overall prevalence of 5.1%. Of these 1,248 women, 778 women were successfully recruited, and 386 and 392 women were randomly allocated to 1-day and 7-day regimens, respectively. Escherichia coli was the most common potentially pathogenic bacteria detected, its prevalence approaching 50%. Bacteriologic cure rates at treatment day 14 were 75.7% and 86.2% for 1-day and 7-day regimens, respectively. The cure rate difference was -10.5% (95% confidence interval -16.1% to -4.9%). Mean birth weight and mean gestational age at delivery were significantly lower in the 1-day regimen group. There were fewer adverse effects in the 1-day regimen group, but the differences were not statistically significant.CONCLUSION: A 1-day regimen of nitrofurantoin is significantly less effective than a 7-day regimen. Women with asymptomatic bacteriuria in pregnancy should receive the standard 7-day regimen.