About 120,000 babies (1 in 33) in the United States are born each year with birth defects (1). A birth defect is an abnormality of structure, function or metabolism (body chemistry) present at birth that results in physical or mental disabilities or death. Several thousand different birth defects have been identified. Birth defects are the leading cause of death in the first year of life (2).
What causes birth defects?
Both genetic and environmental factors, or a combination of these factors, can cause birth defects. However, the causes of about 70 percent of birth defects are unknown (1).
Single gene defects: In many cases, a single gene change can cause birth defects. Every human being has about 20,000 to 25,000 genes that determine traits like eye and hair color (3). Genes also direct the growth and development of every part of our physical and biochemical systems. Genes are packaged into each of the 46 chromosomes inside our cells.
Each child gets half its genes from each parent. A person can inherit a genetic disease when one parent (who may or may not have the disease) passes along a single faulty gene. This is called “dominant inheritance.” Examples include:
Many other genetic diseases are inherited when both parents (who do not have the disease) carry an abnormality in the same gene and pass it on to a child. This is called “recessive inheritance.” Examples include:
There also is a form of inheritance called “X-linked,” in which sons can inherit a genetic disease from a mother who carries the gene (usually with no effect on her own health). Examples include:
- Hemophilia (a blood-clotting disorder)
- Duchenne muscular dystrophy (progressive muscle weakness)
Chromosomal birth defects: Abnormalities in the number or structure of chromosomes can cause many birth defects. Chromosomal abnormalities usually result from an error that occurred when an egg or sperm cell was developing. As a result of this error, a baby can be born with too many or too few chromosomes, or with one or more chromosomes that are broken or rearranged.
Down syndrome, in which a baby is born with an extra chromosome 21, is one of the most common chromosomal abnormalities. Children with Down syndrome have varying degrees of mental retardation, characteristic facial features and, often, heart defects and other problems. Babies born with extra copies of chromosome 18 or 13 have multiple birth defects and often die in the first months of life. Extra copies of most other chromosomes rarely allow survival to live birth and are common causes of miscarriage.
Missing or extra sex chromosomes (X and Y) affect sexual development and may cause infertility, growth abnormalities and behavioral and learning problems. However, most affected individuals live fairly normal lives. Examples include Turner syndrome (in which a girl is missing all or part of an X chromosome) and Klinefelter syndrome (in which a boy has one or more extra X chromosomes).
Environmental factors: Environmental substances that can cause birth defects are called teratogens. These include alcohol, certain drugs/medications, infections, and certain chemicals.
Each year between 1,000 and 6,000 babies are born with fetal alcohol syndrome (FAS) in this country (4). FAS is a pattern of mental and physical birth defects that is common in babies of mothers who drink heavily during pregnancy. Women who are pregnant or planning pregnancy should not drink any alcohol. Even moderate or light
drinking during pregnancy may harm the baby.
Some drugs and medications can contribute to birth defects. For example, the acne drug isotretinoin (sold under the brand names
Accutane, Amnesteem, Claravis and Sotret) poses a high risk of serious birth defects. A woman who is pregnant or who could become pregnant should never use this drug. Illicit drugs such as cocaine also may pose a risk.
Certain infections can result in birth defects when a woman contracts them during pregnancy. About 40,000 babies a year (about 1 percent of all newborns in this country) are born with a viral infection called
cytomegalovirus (CMV) (5). About 1 in 10 infected babies develop serious disabilities, including mental retardation and loss of vision and hearing (5). Pregnant women often get CMV from young children who have few or no symptoms.
Sexually transmitted infections in the mother also can endanger the fetus and newborn. For example, untreated syphilis can result in stillbirth, newborn death or bone defects. About 412 babies were affected by congenital syphilis in 2002(6).
Multi-factorial birth defects: Some birth defects appear to be caused by a combination of one or more genes and environmental exposures. This is called “multi-factorial inheritance.” In some cases, an individual may inherit one or more genes that make him more likely to have a birth defect if he is exposed to certain environmental substances (such as cigarette smoke). These individuals have a genetic predisposition to a birth defect. But if the individual is not exposed to the environmental substance before birth, he probably won't have the birth defect. Examples of multi-factorial birth defects include:
What are the most common birth defects?Cleft lip/palate and
Down syndrome are among the most common birth defects in the United States (7). About 6,800 babies are born with cleft lip/palate each year (7). Cleft lip/palate can cause problems with eating, speech and language. Some affected babies have a small cleft that can be corrected with one surgical procedure, while others have severe clefts and need multiple surgeries. About 5,500 babies are born each year with Down syndrome (7).
About 1,900 babies are born with a serious heart defect called transposition of the great arteries each year (7). Many more babies are born with other serious heart defects. While advances in surgery have dramatically improved the outlook for affected babies, heart defects remain the leading cause of birth defect-related infant deaths (8). Health care providers usually do not know what causes a baby's heart to form abnormally, although both genetic and environmental factors play a role.
Spina bifida (open spine) occurs in about 1,300 babies each year (9). Affected babies have varying degrees of paralysis and bladder and bowel problems. Both genetic and environmental factors (including insufficient amounts of a vitamin called
folic acid) appear to play a role.
Other common birth defects include musculoskeletal defects (including arm and leg defects), gastrointestinal defects (including defects of the esophagus, stomach and intestines) and eye defects (7). These birth defects usually are multi-factorial.
What are birth defects of body chemistry?
In 2002, about 3,000 babies were born with disorders affecting body chemistry (metabolic disorders) (10). These disorders are not visible, but they can be harmful or even fatal.
Most disorders of body chemistry are recessive genetic diseases. These diseases result from the inability of cells to produce enzymes (proteins) needed to change certain chemicals into others, or to carry substances from one place to another. An example is
Tay-Sachs disease. Affected babies lack an enzyme needed to break down certain fatty substances in brain cells. These substances build up and destroy brain cells, resulting in blindness, paralysis and death by age 5.
Another example is
phenylketonuria (PKU). Affected babies cannot process a part of protein, which builds up and damages the brain. Newborn screening tests routinely detect babies with PKU, so they can be placed on a special diet that prevents mental retardation. The March of Dimes recommends that all newborns be
screened for 29 disorders (including
hearing loss) for which effective treatment is available.
Can birth defects be prevented?There are a number of steps a woman can take to reduce her risk of having a baby with a birth defect. One important step is a
preconception visit with her health care provider. During this visit, the provider can identify, and often treat, health conditions that can pose a risk in pregnancy, such as
high blood pressure or
diabetes. The provider can provide advice on lifestyle factors, such as quitting smoking and avoiding alcohol, and occupational exposures that can pose pregnancy risks. The provider also can make sure that any medications a woman takes are safe during pregnancy. All of these steps help prevent birth defects.
A preconception visit is especially crucial for women with chronic health conditions, like diabetes, high blood pressure and epilepsy, which can affect pregnancy. For example, women with diabetes who have poor blood sugar control are several times more likely than women without diabetes to have a baby with a serious birth defect. However, if their blood sugar levels are well controlled starting before pregnancy, they are almost as likely to have a healthy baby as women without diabetes (11).
At a preconception visit, the provider can check to see if a woman's vaccinations are up to date. If she is not immune to
rubella and
chickenpox, she should be vaccinated before pregnancy. With widespread childhood vaccination, rubella is now uncommon. However, if a pregnant woman comes down with the disease, it poses a high risk of birth defects. Chickenpox also can cause birth defects, though the risk is low. A woman should wait for one month after being vaccinated before trying to become pregnant.
The provider also will ask a woman about her health history, as well as that of her partner and her family. This may help the provider identify risk factors for birth defects or inherited genetic conditions. The provider may refer couples with risk factors to a
genetic counselor. A genetic counselor can discuss the risks of birth defects in their children and arrange for
blood tests (such as carrier tests), when needed.
All women who could become pregnant should take a daily multivitamin containing 400 micrograms of the B-vitamin
folic acid. Studies show that taking this vitamin before and during early pregnancy reduces the risk of having a baby with neural tube defects (spina bifida and anencephaly). If a woman already has had a pregnancy affected by one of these birth defects, she should consult her provider before pregnancy about how much folic acid to take. Generally a higher dose, 4 milligrams, is recommended (12). Women with diabetes or epilepsy or who are obese are at increased risk of these birth defects. They should ask their providers before pregnancy about whether they should take the larger dose of folic acid.
A woman who is pregnant or planning pregnancy should avoid drinking alcohol, smoking and using drugs. All of these can cause birth defects and other pregnancy complications. She should not take any medication (prescription, over-the-counter or herbal) without first checking with her health care provider. She should also avoid changing the cat's litter box or eating raw or undercooked meat. These are possible sources of an infection called toxoplasmosis that can cause birth defects.
Can some birth defects be diagnosed before birth?
Some birth defects can be diagnosed before birth using one or more prenatal tests, including
ultrasound,
amniocentesis and
chorionic villus sampling (CVS). Ultrasound can help diagnose structural birth defects, such as spina bifida, heart defects and some
urinary tract defects. Amniocentesis and CVS are used to diagnose or rule out chromosomal abnormalities, such as Down syndrome, and numerous genetic birth defects. Most women have screening tests (blood tests) to see if they are at increased risk of certain birth defects, including Down syndrome and spina bifida. These screening tests cannot diagnose a condition, but they can suggest that further diagnostic testing is needed.
Can birth defects be treated before birth?
A small percentage of couples learn through prenatal diagnosis that their baby has a birth defect. While this news can be devastating, prenatal diagnosis sometimes can improve the outlook for the baby. It is now possible to treat some birth defects before birth. For example, biotin dependence and methylmalonic academia (two life-threatening inherited disorders of body chemistry) have been diagnosed by amniocentesis and treated in the womb, resulting in the births of healthy babies.
Prenatal surgery has saved babies with urinary tract blockages and rare tumors of the lung. More than 300 babies have undergone experimental prenatal surgery to repair spina bifida before birth (13). Prenatal surgery poses a number of serious risks for mother and baby, including
preterm birth. (The National Institutes of Health is currently conducting a study through 2007 to compare the safety and effectiveness of surgery before and after birth for babies with spina bifida. For information contact
www.spinabifidamoms.com.) Doctors also have saved babies with serious heart rhythm disturbances by treating the pregnant woman with medications.
However, even when a fetus has a condition for which prenatal treatment is not yet possible, prenatal diagnosis permits parents to prepare themselves emotionally, and to plan with their provider the safest timing, hospital facility and method of delivery.
Couples who have had a baby with a birth defect, or who have a family history of birth defects, should consider consulting a genetic counselor. These health professionals help families understand what is known about the causes of a birth defect, and the chances of the birth defect recurring in another pregnancy. Genetic counselors can provide referrals to medical experts as well as to appropriate support groups in the community. The
National Society of Genetic Counselors provides the names and contact information of genetic counselors.
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