Overview of Birth Defects
Birth defects, also called congenital anomalies, are physical abnormalities that occur before a baby is born. They are usually obvious within the first year of life.
The cause of many birth defects is unknown, but infections, genetics, and certain environmental factors increase the risk.
After the baby is born, the diagnosis may be based on a physical examination, imaging tests, and blood tests.
Some birth defects can be prevented by maintaining good nutrition while pregnant and avoiding alcohol, radiation, and certain drugs.
Some birth defects can be corrected with surgery or managed with drugs.
Birth defects can involve any part of any organ in the body, including the following:
Some birth defects are more common than others.
Birth defects are the leading cause of death in infants in the United States, and some cause the death of the fetus (miscarriage).
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.
Some Common Birth Defects
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 (see Table: Some Drugs That Can Cause Problems During Pregnancy*), alcohol, nutritional deficiencies, certain infections in the mother, and genetic disorders.
Some risks are avoidable. Others occur no matter how strictly a pregnant woman adheres to healthful living practices. Many birth defects develop before a woman knows she is pregnant.
A teratogen is any substance that can cause or increase the chance of a birth defect. Teratogens include
Most pregnant women who are exposed to teratogens have newborns without abnormalities. Whether a birth defect occurs depends on when, how much, and how long the pregnant woman was exposed to the teratogen (see Exposures During Pregnancy).
Exposure to a teratogen most commonly affects the fetal organ that is developing most rapidly 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.
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. Cleft lip (a separation of the upper lip) or cleft palate (a split in the roof of the mouth) is also more likely to develop.
Obesity in the mother also increases the risk of a neural tube defect.
Chromosomes and genes may be abnormal. These abnormalities may be inherited from the parents, who may either be affected by the abnormalities or who may be carriers of the genes that cause the abnormalities (see Overview of Chromosome and Gene Disorders). Carriers are people who have an abnormal gene for a disorder but who do not have any symptoms of the disorder.
However, many birth defects are caused by chromosome abnormalities or gene mutations in the child. Birth defects caused by genetic factors often include more than just an obvious malformation of a single body part.
Certain infections in pregnant women can cause birth defects (see Infections During Pregnancy). Whether an infection causes a birth defect depends on the age of the fetus at the time of the exposure to the infection.
The infections that most often cause birth defects are
A woman can have one of these infections and not know it because some of these infections can cause few or no symptoms in adults.
Before birth, doctors assess whether a woman is at increased risk of having a baby with a birth defect (see Prenatal Diagnostic Testing). The chance is higher for women who have the following risk factors:
These women may need monitoring and special tests to find out whether their baby is developing normally.
Increasingly, birth defects are being diagnosed before the baby is born.
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 Screening of the Pregnant Woman : Second-Trimester Screening). More recently, doctors are using a test called cell-free fetal DNA analysis. In this test, a sample of a pregnant woman's blood is analyzed to determine whether her fetus has certain genetic disorders. This test is based on the fact that the mother's blood contains a very small amount of DNA (genetic material) from the fetus. This test is called noninvasive prenatal testing (NIPT). NIPT can be used to detect an increased risk of trisomy 21 (Down syndrome), trisomy 13, or trisomy 18 and certain other chromosome disorders. Doctors usually do further testing when an increased risk of a gene abnormality is detected.
After birth, the doctor does a physical examination of the newborn. During this examination, the doctor examines the newborn's skin, head and neck, heart and lungs, and abdomen and genitals and assesses the newborn's nervous system and reflexes. Some newborns have a physical appearance that suggests a certain disorder.