Birth defects are defined as abnormalities of structure, function or body metabolism that are present at birth. Major birth defects are abnormalities that lead to developmental or physical disabilities, or require medical or surgical treatment. There are more than 4,000 different known birth defects, ranging from minor to serious, and although many can be treated or cured, they're the leading cause of death in the first year of life.

According to the March of Dimes, about 150,000 babies are born with birth defects each year in the United States. The American College of Obstetricians and Gynecologists (ACOG) says that 3 out of every 100 babies born in the United States have some kind of major birth defect.http://kidshealth.org/parent/system/ill/birth_defects.html/

Though, the cause of most birth defects is unknown, the use of cigarettes, alcohol, and other drugs, taking of some medicines, and exposure to chemicals and infectious diseases during pregnancy has been linked to birth defects. Researchers are studying the role of these factors, as well as genetics, as causes of birth defects. http://www.cdc.gov/Features/BirthDefects/

There could be a variety of causes causing birth defects, such as:

Genetic problems- when one or more genes don’t work properly or part of a gene is missing.

Problems with chromosomes, such as having an extra chromosome or missing part of a chromosome.

Environmental factors that a woman is exposed to during pregnancy, such as rubella or German measles.http://www.medicinenet.com/birth_defects/article.htm/

Structural or metabolic defects are those in which a specific body part is missing or formed incorrectly. Metabolic birth defects are those in which there is an inborn problem in the body chemistry. The most common type of major structural defects are heart defects, which affect one in 150 babies in the United States. Other common structural defects include spina bifida, cleft palate, clubfoot, and congenital dislocated hip.

Metabolic defects affect one in 3,500 babies and usually involve a missing or incorrectly formed enzyme (a protein necessary for processing chemical substances in the body). Most children with a metabolic birth defect do not have any visible abnormalities, but metabolic defects are usually harmful or can be even fatal. Metabolic defects include Tay-Sachs disease, a fatal disease that affects the central nervous system, and phenylketonuria (PKU), which affects the way the body processes protein.http://kidshealth.org/parent/system/ill/birth_defects.html/

Defects caused by congenital infections result when a mother gets an infection before or during the pregnancy. Infections that can cause birth defects include rubella (German measles), cytomegalovirus (CMV), syphilis, toxoplasmosis, Venezuelan equine encephalitis, parvovirus, and rarely, chickenpox. None of these affect 100 percent of babies whose mothers are infected during pregnancy.

If the mother is infected during early pregnancy, rubella carries the highest risk for birth defects (approximately 20 percent). Because of nearly universal immunization in the United States, rubella is very, very rare and the congenital rubella syndrome is almost never seen. CMV is probably the most common congenital infection and may be associated with intellectual disability (mental retardation) and hearing loss.http://kidshealth.org/parent/system/ill/birth_defects.html/