a) Comparative randomized trial: Considering the prevalence of placenta parasitaemia to be 35% in pregnant women who do not get prophylaxis, 24% in women with weekly CQ prophylaxis, 15% in women with intermittent presumptive treatment with CQ and 5% in women with intermittent presumptive treatment with SP, and using a significance level of 5%, a power of 80% and 20% drop-out, each group should include 200 subjects.