Slow Heart Registry of Fetal Immune-mediated High Degree Heart Block

The SLOW HEART REGISTRY is a multi-centered prospective observational study of fetuses diagnosed with high-degree immune-mediated atrio-ventricular heart block (AVB). 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. The study aims are to document the outcome of patients diagnosed prenatally with immune-mediated 2nd or 3rd degree AVB irrespective of the primary choice of prenatal care. The primary objective will be to determine the rate of transplant-free survival to 1 year of life of fetuses with AVB based on the prenatal management decision: * Cohort 1: Fetuses not treated with fluorinated glucocorticoids * Cohort 2: Fetuses treated with fluorinated glucocorticoids from the time of immune-mediated AVB diagnosis. Secondary objectives will be to determine: 1. the evolution of clinical findings from AVB diagnosis to birth (AV conduction; fetal heart rate; other NL manifestations; fetal growth; effusions/hydrops) between cohorts; 2. the need of new/additional treatment (steroids; beta-mimetics; IVIG) to birth; 3. gestational age and weight at birth; 4. postnatal management (pacing; steroids; IVIG); and 5. clinical evolution from birth to 1-3 years of life (cardiac function; developmental milestones; infant growth; health). Prevalence of relevant fetal-maternal events and complications (death; IUGR; morbidity) between the study cohorts will also be determined. All management is decided by the treating center and physicians in accordance to institutional guidelines and clinical findings. Patient enrollment in the SLOW HEART REGISTRY is possible within up to 8 days of the initial management decision. Participation in this prospective observational cohort study requires site REB approval and an executed legal contract with the primary investigator/SickKids Hospital, Toronto.