Few studies are specifically designed to address health concerns that are already relevant during pregnancy. The consequence is a lack of evidence on best clinical practice. This includes mothers and their babies when pregnancy is complicated by an abnormally slow heart rate due to maternal antibody-mediated heart disease in the unborn baby (fetus). Since the late seventies, it has been possible to detect and monitor fetal disease by ultrasound images and to treat selected conditions with pharmaceuticals administered via the mother. To this day, physicians need to make decisions about the management of such pregnancies without evidence from prospective clinical trials on drug efficacy and safety. The SLOW HEART REGISTRY is a multi-centered prospective observational study that will address the knowledge gap to guide future management of high-degree immune-mediated heart block to the best of care. The study seeks to establish an international database of the management and outcome of affected fetuses, to be used to publish information on the results of currently available prenatal care and to evaluate the need for additional research.