Congenital disorders are those present at birth (birth defects). They fall into two categories: developmental disorders that occur during growth of the fetus and hereditary (familial) disorders that can be passed from parents to children through the germ cells. Genetic disorders are caused by a mutation (change) in the genes or chromosomes of the cells. They may involve changes in the number or structure of the chromosomes or changes in single or multiple genes. The appearance and severity of genetic disorders may also involve abnormal genes interacting with environmental factors. Examples are the diseases that "run in families," such as diabetes mellitus, heart disease, hypertension, and certain forms of cancer. 

A carrier of a genetic disorder is an individual who has a genetic defect that does not appear but that can be passed to offspring. Carriers of some genetic disorders can be identified using laboratory tests. 

Teratogens are factors that cause malformation of the developing fetus. These include infections, such as rubella (German measles), herpes simplex, and syphilis; alcohol; drugs; chemicals; and radiation. The fetus is most susceptible to teratogenic effects during the first 3 months of pregnancy. Examples of developmental disorders are atresia (absence or closure of a normal body opening), anencephaly (absence of a brain), cleft lip, cleft palate, and congenital heart disease. Spina bifida is incomplete closure of the spine, through which the spinal cord and its membranes may project. This usually occurs in the lumbar region. If there is no herniation of tissue, the condition is spina bifida occulta. Protrusion of the meninges through the opening is a meningocele; in a myelomeningocele, both the spinal cord and membranes herniate through the defect.

Many congenital disorders can now be detected before birth. Ultrasonography, in addition to being used to monitor pregnancies and determine fetal sex, can also reveal certain fetal abnormalities. In amniocentesis, a sample is withdrawn from the amniotic cavity with a needle. The fluid obtained can be analyzed for chemical abnormalities. The cells are grown in the laboratory and tested for biochemical disorders. A karyotype is prepared to study the genetic material. In chorionic villus sampling (CVS), small amounts of the membrane around the fetus are obtained through the cervix for analysis. This can be done at 8 to 10 weeks of pregnancy, in comparison with 14 to 16 weeks for amniocentesis.

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