A case of a massive benign intrapericardial teratoma that presented at two weeks of age with significant respiratory distress is described. In retrospect, it was discovered that this infant had been evaluated by fetal echocardiography while in utero at 20 weeks of gestation due to a maternal history of surgically corrected tetralogy of Fallot. The fetal echocardiogram demonstrated normal intracardiac anatomy and no evidence of either a pericardial mass or effusion. It is concluded that this tumor spontaneously enlarged some time between 20 and 40 weeks of gestation.