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Pathology => Human Diseases => Birth Defects
Birth Defects
INTRODUCTION
Birth Defects, also called congenital disorders, any abnormalities of structure or function that are present at birth. The process of fetal development can be disrupted by a variety of external factors such as exposure to radiation, heat, chemicals, infectious agents, or maternal disease. An external agent that alters fetal development is called a teratogen (Greek teratos,"monster"; genes,"born"). Developmental malformations can also be the result of abnormalities in the genetic makeup of the fetus, or they can be caused by a combination of genetic and environmental influences.
Twenty percent or more of malformed fetuses are spontaneously aborted; the rest result in a newborn with a birth defect. Although each single type of birth defect is rare, taken together they make up almost 5 percent of all live births and cause about 20 percent of infant deaths in the period immediately after birth. About one in ten develop mental illnesses is hereditary and arises from an abnormality in a single gene. Another 5 percent of birth defects arise from physical abnormalities in the chromosome.
GENETIC CAUSES
Some genetic disorders have symptoms that are manifest at birth. These disorders may result from mutations in a single gene or from more general chromosomal abnormalities. Many diseases and conditions are inherited in a recessive manner: Neither parent may have the defect even though they both carry the causative gene. When both parents have a dominant gene A and a recessive gene a, their offspring may inherit one of four different combinations: AA, Aa, aA, or aa. If the recessive gene a is defective, the statistical probability is that one in four of the offspring will bear the defective trait. In other congenital disorders the presence of only one copy of the recessive gene is sufficient to cause the condition. See also Genetics.
EXTERNAL CAUSES
Approximately one in ten birth defects is the result of some force or factor that comes from outside the human body. For example, the effects of radiation were demonstrated by the increased incidence of birth defects in the offspring of pregnant Japanese women who were exposed to the atomic bomb in 1945 and of American women who underwent radiation therapy while pregnant. Although the risk from a diagnostic X ray is slight, radiologists recommend having only those X rays immediately necessary during pregnancy. Elevated temperature in a pregnant woman (such as that experienced by sitting in a hot tub) can also lead to birth defects.
The belief that the placenta, which unites the fetus to the maternal uterus, is a protective barrier against chemicals in the mother's blood was tragically disproved in the late 1950s and early 1960s. In many countries children were born with limbs missing or arrested in development after their mothers took the sedative thalidomide, which had not been approved for distribution in the United States. Other known teratogens include alcohol, anticonvulsants, chemotherapeutic agents, cocaine, retinoic acid (a treatment for acne), and the antibiotics streptomycin and tetracycline. In 1955 an outbreak of congenital cerebral palsy in Japan was found to be caused by pregnant women's consumption of fish contaminated with the industrial chemical methylmercury.
A number of infections, when contracted by a pregnant woman, can endanger her unborn child. One of these is rubella, or German measles, which can lead to mental retardation and abnormalities in sight and hearing in the newborn. Vaccination of girls during childhood or adolescence can prevent a later infection during pregnancy. Other maternal infections occurring during pregnancy that can damage a fetus include acquired immune deficiency syndrome (AIDS), chicken pox, toxoplasmosis, and cytomegalovirus.
Women with insulin-dependent diabetes mellitus who are not adequately treated during pregnancy are subject to an increased risk of giving birth to a child with heart defects and a variety of other problems. Poorly controlled phenylketonuria in a pregnant woman can also lead to multiple birth defects and mental retardation in the child.
POLYGENIC AND MULTIFACTORIAL CAUSES
No definite cause is known as yet for about two-thirds of birth defects. Some spine and heart malformations are thought to be polygenic, that is, the result of several abnormal genes that are present simultaneously. Other appear to be multifactorial, resulting from abnormal genes interacting with harmful environmental factors. Some birth defects are found more frequently in children with older parents. It has been found that the risk of Down Syndrome, for instance, rises with the increasing age of the mother.
DETECTION
Scientists have developed several tests for defective genes and for fetal disorders. For some hereditary conditions such as Tay-Sachs disease and sickle-cell anemia, tests are available to detect the defective gene in the healthy parents.
Many diseases can be diagnosed in the fetus by drawing a sample of the amniotic fluid surrounding the fetus and culturing fetal cells for analysis or testing the fluid itself. Another method of obtaining fetal cells for study early in pregnancy is called chorionic villus sampling. The fetal cells can be tested for the presence of a particular function, or their DNA can be scanned for evidence of a genetic disorder. Also, a sample of fetal blood for testing may be obtained directly from the umbilical cord, as may be necessary in cases where DNA diagnosis is not possible.
Some tests for birth defects can be performed on the mother without being as invasive as amniocentesis, chorionic villus sampling, or fetal blood sampling. One of these methods is ultrasonography, which can reveal a number of abnormalities, as well as an accurate determination of fetal age, the presence of multiple pregnancies, and even fetal sex. Elevated levels of alphafetoprotein in a pregnant woman's blood may indicate the presence of spina bifida; depleted levels sometimes indicate the presence of Down syndrome. Unusual concentrations of two other substances in maternal serum-unconjugated estriol and human chorionic gonadotrophin-have also been found when the fetus has Down syndrome.
Many structural birth defects that result from arrested or incomplete development are amenable to corrective surgery. These include cleft lip and palate, failure of development of parts of the digestive tract, and openings between the chambers of the heart. The treatment of hereditary diseases with the techniques of genetic engineering is a novel approach, with great promise for the future, that has already reached the stage of clinical trials.
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