To detect a 20% reduction in the estimated risk of severe anemia (Hb <7 g/dl) among multigravidae, estimated to be 30% in the control group, with 80 % power at the 5% level of significance and allowing for a 30% loss to follow-up it was calculated that a study with 1115 women in each arm was required. To show a reduction in the proportion of low birthweight babies from the expected 18% in the control group to 12% in the SP group with 80% power it was estimated that 2 x 1538 women would be needed. Thus, the target sample size was 3,000.