A birth defect is an abnormality in the body structure or chemistry of a newborn child. It may be caused by hereditary factors (that is, by genetic causes), by environmental influences that affect the embryo or fetus in the mother's womb, or by a combination of factors. Often, the cause of a birth defect is unknown.
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Birth defects sometimes are called congenital anomalies (kon-JEN-i-tal a-NAM-a-leez). Congenital means present at birth, and anomalies are abnormalities or irregularities. An abnormality present at birth usually is not considered a birth defect unless it results in a disease or in a physical or mental disability. For example, birthmarks are seldom considered birth defects because they usually do not cause health problems.
The March of Dimes estimates that 3 to 5 percent of babies born in the United States have some type of birth defect. Some birth defects, such as cleft palate, occur infrequently. Others, such as some congenital heart defects, are more common. Some hereditary defects are more common in certain populations than in others. For example, sickle-cell anemia, an inherited blood disease, occurs mainly in people of African ancestry, whereas Tay-Sachs disease, a fatal disorder of body chemistry, primarily affects people of eastern European Jewish ancestry.
How Do Hereditary Factors Cause Birth Defects?
Each of us has genes * that are inherited from our parents. The genes occur in pairs along threadlike bodies called chromosomes * , which are located in the nucleus * of each cell in the body. Genes determine our inborn characteristics, or traits. These traits include how we look and how the chemical substances in our bodies function. In the case of birth defects, the genes also may determine abnormalities if they are faulty in some way.
Mendelian inheritance patterns
The simplest patterns of birth defect inheritance are called Mendelian, named for the Austrian monk Gregor Mendel who observed them in the nineteenth century. In Mendelian inheritance, traits (including defects) can be transmitted by way of dominant or recessive genes.
It works this way: A child inherits two copies of each gene, one from the mother and one from the father. If a defective gene is dominant, a child who inherits even one copy of it will have the defect. That is because the defective copy "dominates," or overwhelms, the normal copy inherited from the other parent. But if a defective gene is recessive, the child would have to inherit two defective copies—one from the mother and one from the father—in order to have the defect. A person who inherited only one defective copy would be healthy but could pass the defective copy on to his or her own children. These kinds of inheritance patterns cause what researchers call autosomal (aw-to-SO-mal) birth defects.
Examples of autosomal dominant birth defects are Huntington's disease, a nervous system disorder, and Marfan syndrome, which is characterized by tallness, elongated bones, and heart problems. Some birth defects, such as Huntington's disease, may not show symptoms for many years.
Other birth defects are determined by genes located on the X chromosome (the X and Y chromosomes determine the sex of an infant). Such abnormalities are said to be X-linked. Hemophilia, a blood disorder, and color blindness are examples of X-linked birth defects.
* genes are chemicals in the body that help determine a person's characteristics, such as hair or eye color. They are inherited from a person's parents and are contained in the chromosomes found in the cells of the body.
* chromosomes are thread-like structures inside cells on which the genes are located.
* nucleus is the part of the cell that contains its genetic information.
Many hereditary birth defects, however, are not simply dominant, recessive, or X-linked. They may be produced instead by multiple faulty genes.
Some birth defects are caused by extra, missing, incomplete, or misshapen chromosomes. Down syndrome, one of the most common birth defects, usually is caused by the presence of an extra chromosome in the cells. Down syndrome produces mental retardation, short stature, and distinctive facial features. Defects involving the sex chromosomes can produce problems in sexual development, including sterility, which is an inability to have children.
How Do Environmental Factors Cause Birth Defects?
Birth defects also can be caused by environmental factors, either alone or together with faulty genes. "Environmental" here refers to the environment in the mother's uterus, or womb, rather than to the earth's environment. However, scientists are studying the possible influence on birth defects of poisons in the earth's environment.
Pregnant women who consume excessive amounts of alcohol during the early stages of pregnancy risk having babies with fetal alcohol syndrome. Children with this disorder may have various defects in growth, facial appearance, and mental ability. Scientists are studying whether even moderate use of alcohol can damage a fetus. Smoking during pregnancy increases the likelihood that the baby will have lower than normal weight at birth, increasing the risk of defects.
Some illnesses in a pregnant woman can cause damage to the fetus. For example, German measles (rubella) can cause deafness, blindness, and heart defects in the newborn. Sexually transmitted diseases also can be transmitted to the fetus or to the newborn at birth.
Certain medications have been linked to birth defects. The most famous is the drug thalidomide (tha-LID-o-mide), a sedative that in the early 1960s was found to cause greatly shortened arms and legs in many newborn infants. Many other medications, including tranquilizers and antibacterial and anticancer drugs, can cause congenital abnormalities.
Other environmental factors believed to increase the risk of birth defects include poor nutrition and the age of the mother. For example, the older a pregnant woman is, the more likely she is to give birth to a child with Down syndrome. If the expectant mother is 35 or older, experts recommend that the fetus be tested.
How Do Doctors Diagnose Birth Defects?
Some birth defects can be diagnosed while the child is still in its mother's womb. A procedure called ultrasound, which uses sound waves to produce an image of a fetus on a screen, can detect some malformations. For example, a defect called spina bifida, in which a part of the spinal cord is exposed, can be discovered by ultrasound.
The U.S. and the World
- An estimated 515,000 people worldwide died of birth defects in 1998. Almost 93 percent of those deaths were in low-and middle-income nations.
- More than 28 million people worldwide live with birth defects, and about 93 percent live in countries with low or middle income.
- Birth defects are the leading cause of infant mortality in the United States. In 1997,6,178 children died before their first birthdays because they had one or more birth defects. That number is 22 percent of all children who died underage 1.
- The most common birth defects involve the heart. About 28 percent of the children under age 1 who died of birth defects in 1997 had problems with their hearts. The second most common category comprised defects of the respiratory system, accounting for about 16 percent of the deaths that year.
- The death rate from birth defects in the United States dropped 37.7 percent between 1979 and 1997. One reason for the decrease is improved medical care for infants with birth defects.
- Children with birth defects who survive their first year still may die of complications from the defects later in life. In 1997,11,912 people of all ages died of birth defects, with only about half under age 1.
- About 150,000 babies a year are born with a birth defect in the United States. The rate in the mid-1990s put the United States twenty-fifth in the world, with twice as many birth defects per 1,000 live births as the nation with the lowest rate, Japan.
In a procedure called amniocentesis (am-nee-o-sen-TEE-sis), a small sample of fluid surrounding the fetus is removed through a needle and examined. This test is useful in detecting inborn metabolic (body chemistry) defects and abnormalities in the chromosomes.
Many birth defects can be diagnosed by a doctor's physical examination of a newborn baby. Other tests, including x-rays, may be ordered if doctors suspect a birth defect; blood tests can detect certain disorders of the blood or body chemistry. Many infants with defects can develop normally if they receive prompt treatment.
How Do Doctors Treat Birth Defects?
Not every birth defect affects the quality of life of the person who has it. Some birth defects have little effect, except perhaps on appearance.
Several birth defects can be treated to prevent or reduce their harmful effects. Surgeons can perform operations to correct such malformations as clubfoot, cleft palate, cleft lip, and structural defects in the heart and digestive tract. Treatment can lessen the symptoms of cystic fibrosis, an inherited disease that interferes with breathing. In some cases, disorders such as hydrocephalus (hy-dro-SEF-a-lus), a damaging buildup of fluid in the head, can be remedied even before birth.
Doctors can sometimes treat inborn disorders of body chemistry with medications and special diets. For example, prompt treatment can prevent brain damage in phenylketonuria (fen-il-kee-to-NOOR-ee-a), or PKU, a metabolic defect that can produce severe mental retardation. Special education, rehabilitation, and the use of special devices and machines can help to offset some mental and physical handicaps, such as blindness and deafness.
Thalidomide is a sedative, a medication prescribed to calm the nerves. If taken in the early months of pregnancy it can cause a birth defect called phocomelia (fo-ko-MEE-lee-a), in which the arms, legs, or both are very short and underdeveloped.
Thalidomide was prescribed for pregnant women in Europe in the late 1950s and early 1960s until it was linked to phocomelia in thousands of newborn babies. Fortunately for Americans, thalidomide never was sold in the United States because the U.S. Food and Drug Administration refused to approve it. An employee of this federal agency, Dr. Frances Kelsey, insisted that thalidomide's safety had not been proven. Many people think Dr. Kelsey saved hundreds or even thousands of children from severe birth defects.
The thalidomide tragedy led to an increase in government control over the marketing of drugs throughout the world, and the use of thalidomide is now banned or strictly controlled in most countries.
Thalidomide may prove helpful in the treatment of such serious diseases as AIDS and leprosy, and researchers are continuing to study its therapeutic uses.
How Do Doctors and Parents-to-Be
Prevent Birth Defects?
No one can guarantee that a baby will be born "perfect" and healthy. However, there are ways to minimize the likelihood of having a child with a
- Pregnant women should not smoke or drink alcoholic beverages, nor should they use drugs of any kind unless prescribed by a doctor.
- Certain vitamins, if taken in proper amounts by the mother-to-be, can help prevent some birth defects. For example, folate (folic acid) taken during pregnancy can help prevent certain defects of the spinal column and central nervous system, including spina bifida.
- Vaccination well before pregnancy can prevent birth defects that might occur if the mother were to develop German measles while pregnant.
Couples should seek genetic counseling before planning to have a child if the mother, father, or relatives have hereditary abnormalities.
Deafness and Hearing Loss
Fetal Alcohol Syndrome
German Measles (Rubella)
Sexually Transmitted Diseases
Kidd, J. S., and R. A. Kidd. Life Lines: The Story of the New Genetics. New York: Facts on File, 1999. This survey for young adults discusses the evolution of the study of genetics.
Marshall, Elizabeth L. The Human Genome Project: Cracking the Code Within Us. New York: Franklin Watts, 1997. A look at this international project and the scientists doing the research.
U.S. Centers for Disease Control and Prevention (CDC). CDC posts several
fact sheets about birth defects at its website.
The March of Dimes Birth Defects Foundation, 1275 Mamaroneck Avenue,
White Plains, NY 10605. The mission of this national organization is to
improve the health of babies by preventing birth defects and infant
mortality. Its website offers information on the cause and prevention of