Birth defects are defined as abnormalities of structure, function, or body metabolism that are present at birth and that lead to mental or physical disabilities or death. Presently there are more than 4,000 known birth defects. Some are minor and can be treated or cured. Others are major and cause medical problems that are lifelong. Some birth defects cause death within the first year of life. Some birth defects have no known cause while other s are caused by infections that the mother contracted during pregnancy, still others are caused by alcohol use by the mother. The March of Dimes estimates that 60% of birth defects occur for unknown reasons. Birth defects that are structural are those that result from a specific body part missing or being formed improperly, such as with a heart defect or spina bifida. Metabolic defects occur in approximately 1 in 3500 births. These defects typically involve a missing enzyme that affects the central nervous system such as occurs with Tay-Sachs or PKU. Other birth defects are caused by an infection during pregnancy, such as rubella or syphilis. Rubella carries the highest risk of causing birth defects. Some birth defects are multi-factorial ﾖ they are caused by more than one cause. Perhaps the mother was exposed to chemicals during her pregnancy and there is a faulty gene involved. Regardless, birth defects are lifelong health conditions.