Autoantibody-mediated autoimmune diseases sometimes can be transmitted transplacentally, as in the case of neonatal Grave’s disease or congenital complete heart block and neonatal lupus. IgG antibodies/autoantibodies can cross the placenta, whereas IgM cannot. Thus, neonatal autoimmune diseases are invariably caused by IgG, not IgM, autoantibodies. In view of the half-life of IgG (twenty-one to twenty-eight days), nearly all maternal IgG disappears from the circulation of the baby by six to twelve months postpartum. Thus, in most cases, neonatal autoimmune disease is transient. One exception is congenital complete heart block, which is thought to be mediated by the transplacental passage of anti-Ro or anti-La autoantibodies that cross-react with cardiac antigens, causing permanent inflammation-mediated damage to the cardiac conduction system.