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BIRTH DEFECTS SYMPTOMS |
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The specific cause of many congenital
abnormalities is unknown, but several factors associated with
pregnancy and delivery can increase the risk of birth defects. Teratogens Any substance that can causes abnormal development of the egg in the mother's womb is called a teratogen. In the first two months after conception, the developing organism is called an embryo; developmental stages from two months to birth are called fetal. Growth is rapid, and each body organ has a critical period in which it is especially sensitive to outside influences. About 7% of all congenital defects are caused by exposure to teratogens. Drugs Only a few drugs are known to cause birth defects, but all have the potential to cause harm. Thalidomide is known to cause defects of the arms and legs; several other types also cause problems. Alcohol. Drinking large amounts of alcohol while pregnant causes a cluster of defects called fetal alcohol syndrome, which include mental retardation, heart problems, and growth deficiency. Antibiotics. Certain antibiotics are known tetratogens. Tetracycline affects bone growth and discolors the teeth. Drugs used to treat tuberculosis can lead to hearing problems and damage to a nerve in the head (cranial). Sulfa drugs are associated with abnormally high levels of bilirubin in the newborn, which can cause death. Anticonvulsants. Drugs given to prevent seizures can cause serious problems in the developing fetus, including mental retardation and slow growth. Antipsychotic and antianxiety agents. Several drugs given for anxiety and mental illness are known to cause specific defects. Antineoplastic agents. Drugs given to treat cancer can cause major congenital malformations, especially central nervous system defects. They may also be harmful to the health care worker who is giving them while pregnant. Hormones. Male hormones may cause masculinization of a female fetus. A synthetic estrogen (DES) given in the 1940s and 1950s causes an increased risk of cancer in the adult female children of the mothers who received the drug. Recreational drugs. Drugs such as LSD have been associated with arm and leg abnormalities and central nervous system problems in infants. Crack cocaine has also been associated with birth defects. Since drug abusers tend to use many drugs and have poor nutrition and prenatal care, it is hard to determine the effects of individual drugs. Chemicals Environmental chemicals such as fungicides, food additives, and pollutants are suspected of causing birth defects, though this is difficult to prove. Radiation Exposure of the mother to high levels of radiation can cause small skull size (microcephaly), blindness, spina bifida, and cleft palate. How severe the defect is depends on the duration and timing of the exposure. Infections Three viruses are known to harm a developing baby: rubella, cytomegalovirus (CMV), and herpes simplex. Toxoplasma gondii, a parasite that can be contracted from undercooked meat, from dirt, or from handling the feces of infected cats, causes serious problems. Untreated syphilis in the mother is also harmful. Genetic factors A gene is a tiny, invisible unit containing information (DNA) that guides how the body forms and functions. Each individual inherits tens of thousands of genes from each parent, arranged on 46 chromosomes. Genes control all aspects of the body, how it works, and all its unique characteristics, including eye color and body size. Genes are influenced by chemicals and radiation, but sometimes changes in the genes are unexplained accidents. Each child gets half of its genes from each parent. In each pair of genes one will take precedence (dominant) over the other (recessive) in determining each trait, or characteristic. Birth defects caused by dominant inheritance include a form of dwarfism called achondroplasia; high cholesterol; Huntington's disease, a progressive nervous system disorder; Marfan syndrome, which affects connective tissue; some forms of glaucoma, and polydactyly (extra fingers or toes). If both parents carry the same recessive gene, they have a one-in-four chance that the child will inherit the disease. Recessive diseases are severe and may lead to an early death. They include sickle cell anemia, a blood disorder that affects blacks, and Tay-Sachs disease, which causes mental retardation in people of eastern European Jewish heritage. Two recessive disorders that affect mostly whites are: cystic fibrosis, a lung and digestive disorder, and phenylketonuria (PKU), a metabolic disorder. If only one parent passes along the genes for the disorder, the normal gene received from the other parent will prevent the disease, but the child will be a carrier. Having the gene is not harmful to the carrier, but there is the 25% chance of the genetic disease showing up in the child of two carriers. Some disorders are linked to the sex-determining chromosomes passed along by parents. Hemophilia, a condition that prevents blood from clotting, and Duchenne muscular dystrophy, which causes muscle weakness, are carried on the X chromosome. Genetic defects can also take place when the egg or sperm are forming if the mother or father passes along some faulty gene material. This is more common in older mothers. The most common defect of this kind is Down syndrome, a pattern of mental retardation and physical abnormalities, often including heart defects, caused by inheriting three copies of a chromosome rather than the normal pair. A less understood cause of birth defects results from the interaction of genes from one or both parents plus environmental influences. These defects are thought to include: Cleft lip and palate, which are malformations of the mouth Clubfoot, ankle or foot deformities. Spina bifida, an open spine caused when the tube that forms the brain and spinal chord does not close properly. Water on the brain (hydrocephalus), which causes brain damage. Diabetes mellitus, an abnormality in sugar metabolism that appears later in life. Heart defects. Some forms of cancer. A serious illness in the mother, such as an underactive thyroid, or diabetes mellitus, in which her body cannot process sugar, can also cause birth defects in the child. An abnormal amount of amniotic fluid may indicate or cause birth defects. Amniotic fluid is the liquid that surrounds and protects the unborn child in the uterus. Too little of this fluid can interfere with lung or limb development. Too much amniotic fluid can accumulate if the fetus has a disorder that interferes with swallowing. |
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