Routine iron and folate supplementation during pregnancy is often recommended in developing countries, but is not based on evidence from controlled trials. The objective of this study was to determine the effects on pregnancy outcomes of iron-folate supplementation of pregnant women with a hemoglobin level of 9.0 g/dL or more. The authors performed a trial in prenatal clinics in Dhaka, Bangladesh. Eligible women were alternatively assigned to iron-folate or vitamin B (without vitamin B12) supplements from 24 weeks of gestation until the end of the pregnancy. The supplements were identical in appearance. A total of 2007 women were included: 975 in the iron-folic acid group, and 1032 in the vitamin B group. A total of 772 women supplemented with iron-folate and 812 women supplemented with vitamin B were followed up until the end of pregnancy. After delivery, the mean hemoglobin concentration was higher (p<0.01) in the iron-folate group (11.2g/dl) than in the vitamin B group (10.5g/dL). There was a significant increase (odds ratio 1.43; confidence interval 1.08-1.89) of preterm (<37 weeks) delivery in the iron-folate group and 57 g reduction (p=0.02) in the mean birthweight of the singletons born to mothers in the iron-folate group compared to those born in the vitamin B group. No significant difference between the two groups was found in the frequency of low birth weight (<2500g), maternal hypertension, antepartum hemorrhage, maternal infection, stillbirth, and neonatal death. The results suggest that routine iron-folate supplementation during pregnancy might not be beneficial.