Introduction
Birth defects, also called congenital anomalies, are physical abnormalities that occur before a baby is born; they are usually obvious at birth or by 1 year of age.
Birth defects can involve any part of any organ in the body. Some birth defects are more common than others. Birth defects are the leading cause of death in infants in the United States. A birth defect is evident in about 7.5% of all children by age 5 years, although many of these are minor. Major birth defects are evident in about 3 to 4% of newborns. Several birth defects can occur together in the same infant.
 See the table Other Birth Defects.
Causes and Risks
It is not surprising that birth defects are fairly common, considering the complexities involved in the development of a single fertilized egg into the millions of specialized cells that constitute a human being. Although the cause of most birth defects is unknown, certain genetic and environmental factors increase the chance of birth defects developing. These factors include exposure to radiation, certain drugs (for example, isotretinoin, which is used to treat severe acne), alcohol, nutritional deficiencies, some infections in the mother, injuries, and hereditary disorders. Some risks are avoidable. Others occur no matter how strictly a pregnant woman adheres to healthful living practices.
Exposure to Harmful Substances (Teratogens): A teratogen is any substance that can cause or increase the chance of a birth defect. Radiation (including x-rays), certain drugs, and toxins (including alcohol) are teratogens. Most pregnant women who are exposed to teratogens have newborns without abnormalities. Whether or not a birth defect occurs depends on when, how much, and how long the pregnant woman was exposed to the teratogen. Exposure to a teratogen most commonly affects the fetal organ that is developing at the time of exposure. For example, exposure to a teratogen during the time that certain parts of the brain are developing is more likely to cause a defect in those areas than exposure before or after this critical period. Many birth defects develop before a woman knows she is pregnant.
Nutrition: Keeping a fetus healthy requires maintaining a nutritious diet. For example, insufficient folic acid (folate) in the diet increases the chance that a fetus will develop spina bifida or other abnormalities of the brain or spinal cord known as neural tube defects (see Section 22, Chapter 256). Maternal obesity also increases the risk of a neural tube defect.
Genetic and Chromosomal Factors: Chromosomes and genes may be abnormal. These abnormalities may be inherited from the parents, who can be affected by the condition or who can be carriers without symptoms (see Section 1, Chapter 2). However, many birth defects are caused by seemingly random and unexplained changes (mutations) in the genes of the child. Most birth defects caused by genetic factors include more than just the obvious malformation of a single body part.
Infections: Certain infections in pregnant women can cause birth defects. Whether an infection causes a birth defect depends on the age of the fetus. The infections that most often cause birth defects are cytomegalovirus, herpesvirus, parvovirus (fifth disease), rubella (German measles), varicella (chickenpox), toxoplasmosis (which can be transmitted in cat litter), and syphilis. A woman can have such an infection and not know it, because these infections can produce few or no symptoms in adults.
Diagnosis
During pregnancy, doctors assess whether a woman is at increased risk of having a baby with a birth defect (see Section 22, Chapter 256). The chance is higher for women who are older than 35 years, have had frequent miscarriages, or have had other children with chromosomal abnormalities, birth defects, or who died for unknown reasons. These women may need special tests to find out if their baby is normal.
A prenatal ultrasound can often detect specific birth defects. Sometimes blood tests can also help; for example, a high level of alpha-fetoprotein in the mother's blood may indicate a defect of the brain or spinal cord (see Section 22, Chapter 256). Amniocentesis (removing fluid from around the fetus) or chorionic villus sampling (removing tissue from the sac around the developing baby) may be necessary to confirm a suspected diagnosis. Increasingly, birth defects are being diagnosed before the baby is born.
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