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We undertook this study to compare the effectiveness and safety of antenatal daily and weekly supplementation with iron, folic acid, and vitamin B(12) in healthy, pregnant women who were not anemic at gestational week 20.Women with singleton pregnancies and blood hemoglobin (Hb)115 g/L at gestational week 20 (equivalent to 105 g/L at sea level) were randomly assigned to two groups, one consuming one tablet containing 60 mg iron, 200 mug folic acid and 1 mug vitamin B(12) daily (DS, n = 56); the other consuming two tablets once weekly (WS, n = 60). Blood Hb and serum ferritin concentrations were measured every 4 weeks from weeks 20 to 36, and pregnancy outcomes were evaluated.Mild anemia and hypoferritinemia throughout pregnancy occurred less frequently in DS than WS. None of the 116 women had Hb concentrations103 g/L at any evaluation point. In contrast, hemoconcentration (Hb145 g/L) from gestational week 28 onwards occurred in 11% in DS and 2% in WS. We observed ex post facto that hemoconcentration at gestational week 28 was associated with a significantly higher relative risk of low birth weight (RR 6.23, 95% CI 1.46-26.57) and premature delivery (RR 7.78, 95% CI 1.45-24.74).In women who were nonanemic at gestational week 20, both schemes (DS and WS) prevented the occurrence of Hb levels100 g/L. DS women had a higher incidence of hemoconcentration. Hemoconcentration was associated with increased risk of low birth weight and premature delivery.