Birth defects are defined as abnormalities of structure, function or body metabolism that are present at birth. Often these lead to mental or physical disabilities that are lifelong and create many hardships on the family and the individual. There is not a known cause for all birth defects, often they just occur and there are no medical answers. With over 4000 known different types of birth defects that range from minor to severe, some treatable and some not. Birth defects can be caused by genetic malfunctions, environmental causes, drugs or alcohol use, Rh blood factors, or unknown factors. According to the March of Dimes approximately 60% of birth defects are due to unknown causes. Congenital defects result when a mother gets an infection before or during pregnancy. Some infections that can lead to birth defects include rubella, CMV, syphilis, toxoplasmosis, parvovirus, and, although it is rarely, chickenpox. Of these, rubella carries the highest risk for causing birth defects. Genetics play some part in birth defects. Children inherit genes from each parent and even with healthy parents the child can receive genes that have been passed down from previous generations.