Low birth-weight is a leading health problem in developing countries. In a randomized controlled trial, the effect of antimicrobials in pregnant women on improving birth-weight and duration of gestation was evaluated. Two hundred twenty-four pregnant women in their second trimester were randomized to receive metronidazole (200 mg 3 times daily for 7 days) and cephalexin (500 mg twice daily for 5 days) orally by one group. The mean (±SD) birth-weights were 2,545 g (±374) and 2,584 g (±358,

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... ), the low birth-weight rates (<2.5 kg) were 40% and 36% (p=0.28), and the prematurity rates were 8% and 11% (p=0.6) in the treated group and the control group respectively. Due to small sample size, it is cautiously concluded that routine antimicrobials for genital and urinary tract infections of pregnant women do not improve birth-weight or duration of gestation. Rather an unexpected observation was the proportion requiring caesarian section or forceps, which was five-fold higher in the treated group (p=0.001), and given no plausible explanations, this finding needs confirmation. Stunted mothers (<25th centile or 146.4 cm) had twofold higher risk for low birth-weight (p=0.04) and assisted delivery (p=0.1). Low maternal body mass index (<25th centile or 18) had six-fold higher risk for stillbirth or abortion (p=0.007), and high body mass index (>75th centile or 21.2) had threefold higher risk for assisted delivery (p=0.003).