Metronidazole is the recommended treatment for trichomoniasis and bacterial vaginosis in nonpregnant women. Reluctance to use this drug in pregnancy is related to concerns about possible adverse effects on the fetus. However, recent literature suggests associations between these infections during pregnancy and risk of preterm delivery; if such associations are causal and preterm birth is shown to be prevented by effective treatment, the risk-benefit equation may be altered. This article reviews the literature on the carcinogenic and teratogenic potential of metronidazole and discusses the evidence that associates trichomoniasis and bacterial vaginosis with perinatal outcomes. Current practice recommendations are summarized.