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Birth defect


An abnormality of structure, function, or body metabolism present at birth that results in physical or mental disability, or death. Birth defects may range from mild to severe, and some may be life threatening; they may leave a child unable to walk, to hear, to think, or to fight off disease. Birth defects are the leading cause of death in the first year of life. About 150,000 babies are born in the United States each year with birth defects. Birth defects may occur in babies anywhere in the world, to families of any nationality. While the normal risk of birth defects in any pregnancy is about three or four percent, in families with a genetic history of birth defects the risk is much higher. The rate of birth defects–associated deaths declined over the past decade, which may reflect improvements in medical and surgical care, better prenatal diagnosis and subsequent pregnancy termination, and underreporting of deaths associated with birth defects. Birth defects were the leading cause of infant mortality, accounting for about 20 percent of all infant deaths, but a substantial percentage of deaths of older children were also related to birth defects. Types About three or four out of every 100 babies are born with at least one of the more than 4,000 known types of birth defects. Birth defects of the heart and circulatory system affect more infants than any other type; about one in 115 babies born each year has heart or circulatory defects. The next most common type of birth defect include: • problems with muscles or the skeleton, one of every 130 births • genital and urinary tract problems, one in 135 • nervous system and eye problems, one in 235 • chromosome syndromes, one in 600 • club foot, one in 735 • Down syndrome, one in 900 • respiratory tract problems, one in 900 • cleft lip or palate, one in 930 • spina bifida, one in 2,000 • metabolic disorders, one in 3,500 • anencephaly, one in 8,000 • PKU, one in 12,000 When a baby has a structural birth defect, some part of the body (internal or external) is missing or malformed. Heart defects are the most common type of structural birth defect. While advances in surgical treatment have dramatically improved the outlook for babies with heart defects, these remain the leading cause of birth defect–related infant deaths. Doctors usually do not know what causes a baby’s heart to form abnormally, although genetic and environmental factors are believed to play a role. SPINA BIFIDA affects one in 2,000 babies. Every baby’s spine is open when it first forms, but it normally closes by the 29th day after conception. In cases of spina bifida it fails to close. Affected babies suffer varying degrees of paralysis and bladder and bowel problems. Both genetic and nutritional factors appear to play a role in spina bifida. About one baby in 135 has a structural defect involving the genitals or urinary tract. These range from abnormal placement of the urinary opening in males (hypospadias) to absence of both kidneys. While hypospadias are correctable, babies born without any kidneys die in the first hours or days of life. Metabolic disorders affect one in 3,500 babies. Most are recessive genetic diseases that can be fatal if unrecognized and untreated. These diseases are caused by the inability of cells to produce an enzyme needed to change certain chemicals into others, or to carry substances from one place to another. For example, babies with TAY-SACHS DISEASE lack an enzyme needed to break down fatty substances in brain and nerve cells. These substances build up and destroy brain and nerve cells, resulting in blindness, paralysis, and death by age five. PKU (PHENYLKETONURIA) is another metabolic disorder in which affected babies cannot process part of a protein that builds up in blood and causes brain damage. Rubella (GERMAN MEASLES) is probably the best known congenital infection that can cause birth defects. If a pregnant woman is infected in the first trimester, her baby has a one in four chance of being born with one or more symptoms of congenital rubella syndrome (featuring deafness, mental retardation, heart defects, or blindness). It also can cause stillbirth. Fortunately, with widespread vaccination, this syndrome is now rare in this country. The most common congenital viral infection is CYTOMEGALOVIRUS (CMV). About 40,000 babies a year in this country are infected, though only about 10 percent of them actually have symptoms, including mental retardation, vision and hearing loss. Pregnant women often acquire CMV from young children, who usually 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 birth defects involving the bones. About one baby in 2,000 is born with congenital syphilis. Other causes of birth defects include FETAL ALCOHOL SYNDROME, which affects one baby in 1,000 with a pattern of mental and physical birth defects common in babies of mothers who drink heavily during pregnancy. Even moderate or light drinking during pregnancy can pose a risk to the baby. Rh disease of the newborn refers to an incompatibility between the blood of a mother and her fetus, affects about 4,000 infants a year. It can result in JAUNDICE, anemia, brain damage, and death. Rh disease usually can be prevented by giving an Rh-negative woman an injection of a blood product called immunoglobulin at 28 weeks of pregnancy and after the delivery of an Rh-positive baby. However, not all women who can benefit from this treatment get it, and a few cannot benefit from treatment. Babies of mothers who use cocaine early in pregnancy are at increased risk of birth defects and are five times more likely to be born with urinary tract defects than babies of women who do not use cocaine. Causes While birth defects are due to many different factors, the underlying cause is known in only about 40 percent of cases. Some of these known causes are inherited genetic defects, abnormal chromosomes, environmental problems, harmful chemicals, or infections in the pregnant mother. Hereditary birth defects are not uncommon. A single abnormal gene can cause birth defects. Every human being has about 100,000 genes that determine traits like eye and hair color, as well as growth and development. Genes are packed into each of the 46 chromosomes inside cells. Children get half their genes from each parent. Occasionally, a child may inherit a genetic disease when one parent who has the disease passes along a single faulty gene. This is called dominant inheritance. Examples of birth defects caused by dominant inheritance include achondroplasia, a form of dwarfism, and MARFAN SYNDROME, a connective tissue disease. More often, a child inherits a genetic disease only if both healthy parents pass along the same faulty gene. This is called recessive inheritance. Examples include Tay-Sachs disease (a fatal disorder seen mainly in people of Eastern European Jewish heritage) and CYSTIC FIBROSIS (a fatal disorder of lungs and other organs affecting mainly Caucasians of northern European lineage). In X-linked inheritance, sons can inherit a genetic disease from a healthy mother who carries the gene. Examples include hemophilia and Duchenne MUSCULAR DYSTROPHY. Abnormalities in the number or structure of chromosomes can cause numerous birth defects. Due to an error that occurred when an egg or sperm cell was developing, a baby can be born with one too many or one 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. Babies also can be born with extra copies of chromosome 18 or 13, which causes multiple birth defects that are usual fatal in the first months of life. In addition, birth defects may be caused by environmental factors such as drug or alcohol abuse, infections such as German measles (rubella) or cytomegalovirus, or exposure to chemicals such as the acne drug Accutane. Birth defects may appear to be caused by a combination of one or more genes and environmental factors (called multifactorial inheritance), which may result in cleft lip/palate, clubfoot, and some heart defects. Testing for Birth Defects The March of Dimes believes that every state should require testing for at least these 10 disorders: • medium-chain acyl-coa dehydrogenase (M-CAD) deficiency (Incidence: 1 in 15,000) • phenylketonuria (PKU) (1 in 12,000) • congenital hypothyroidism (1 in 4,000) • congenital adrenal hyperplasia (CAH) (1 in 5,000) • biotinidase deficiency (1 in 70,000) • maple syrup urine disease (1 in 250,000) • galactosemia (1 in 50,000) • homocystinuria (1 in 275,000) • sickle cell anemia (1 in 400 among African-Americans; 1 in 1,000–30,000 among Hispanics) • hearing problems These diseases all have reliable screening and diagnostic tests, and there are effective treatments for all of them. However, if these defects go untreated, the consequence is almost inevitably serious. In most states that require testing, parents are not asked for consent. Treatments More and more babies’ lives are being saved because of surgery that corrects birth defects before a baby is born, neonatal intensive care units that provide specialized care to rescue babies, and new tests and treatments to make babies healthier. One of the weapons in the fight against birth defects is gene therapy. Scientists have recently successfully corrected two genetic diseases in mice (HEMOPHILIA and retinitis pigmentosa, a leading cause of blindness). These successes in the laboratory are a first big step toward curing these disorders in humans.