Background The burden of pre-existing cardiovascular disease and the contribution to adverse pregnancy outcomes are not robustly quantified, particularly in low-income countries. We aimed to determine both the prevalence of maternal heart disease through active case finding and its attributable risk to adverse pregnancy outcomes. Methods We conducted a 24-month prospective longitudinal investigation in three Ugandan health centres, using echocardiography for active case finding during antenatal care. Women with and without heart disease were followed to 6 weeks post partum to determine pregnancy outcomes. Prevalence of heart disease was calculated. Per cent attributable risk estimates were generated for maternal, fetal and neonatal mortality. Results Screening echocardiography was performed in 3506 women. The prevalence of heart disease was 17 per 1000 women (95% CI 13 to 21); 15 per 1000 was rheumatic heart disease. Only 3.4% of women (2/58) had prior diagnosis. Cardiovascular complications occurred in 51% of women with heart disease, most commonly heart failure. Per cent attributable risk of heart disease on maternal mortality was 88.6% in the exposed population and 10.8% in the overall population. Population attributable risk of heart disease on fetal death was 1.1% and 6.0% for neonatal mortality Conclusions Occult maternal heart disease may be responsible for a substantial proportion of adverse pregnancy outcomes in low-resource settings. Rheumatic heart disease is, by far, the most common condition, urging global prioritisation of this neglected cardiovascular disease.