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Genetic Testing - Prenatal Genetic Testing

age chromosome differences fetus risk chromosome abnormality

Prenatal (before birth) genetic testing refers to testing the fetus for a potential genetic condition. The pregnant woman is considered the patient and makes decisions regarding prenatal testing. There are a variety of circumstances under which a woman might be offered prenatal genetic testing. The parents of the fetus may have a genetic disorder, or they may be what is known as carriers. An abnormality or birth defect may be detected on ultra-sound that could indicate a genetic condition. The fetus may be identified to be at increased risk for a chromosome abnormality, such as Down syndrome, or a birth defect, such as spina bifida, based on the result of a maternal serum screening test performed on the mother. This is a test that looks at several proteins made by the fetus that are found in a woman's bloodstream while she is pregnant. Or, the mother might be at increased risk for having a baby with a chromosome abnormality because of her age. While all women are at risk for having a baby with a chromosome abnormality, women who are age thirty-five or older are offered prenatal chromosome testing because the chance their fetus has a chromosome abnormality is equal to or higher than the chance she will have a miscarriage due to the sampling procedure.

As with most genetic testing, prenatal genetic testing should occur in conjunction with genetic counseling. The genetic counselor provides supportive, nondirective counseling and information. Nondirective counseling means that while the counselor will try to facilitate decisions regarding testing and future pregnancy management, she will not make specific recommendations. Because the decision to undergo testing is personal and must take into account differences in beliefs, life circumstances, and the risk of the procedure, the decisions regarding testing and pregnancy management must be made by the patient. This encounter is also likely to include information about risk of the fetus being affected, the disorder in question, and available testing options.

Most genetic tests are performed on tissue or a blood sample. For obvious reasons, obtaining a sample from a fetus is not the same as obtaining one from a child or adult. Prenatal testing procedures are invasive, and there is a risk of miscarriage with every procedure. For this reason, specially trained physicians perform these tests. Prenatal testing can be accomplished using three different methods: amniocentesis, chorionic villus sampling, and percutaneous umbilical blood sampling. These tests differ in the type of fetal tissue studied, the timing of the testing during pregnancy, and in their risks and benefits.

Amniocentesis is the most common, and it carries the lowest risk of miscarriage (about one in two hundred pregnancies). It is typically performed between sixteen and eighteen weeks into the pregnancy and involves collecting a small amount of amniotic fluid that contains cells of the developing fetus, which can be used for testing. Chorionic villus sampling is performed earlier than amniocentesis, typically between ten and twelve weeks of pregnancy, but about one in one hundred pregnancies are miscarried as a result of this procedure. It involves obtaining a small sample of chorionic villi (fingerlike projections of the chorion, a membrane that will later develop into the placenta), which should contain cells of the fetus. Percutaneous umbilical blood sampling, typically performed after eighteen This genetic test is positive for Trisomy 18. Fluorescence in situ hybridization reveals three copies of chromosome 18 (green) and female sex chromosomes (fuschia) within a nucleus (blue). weeks, is the most difficult to perform and carries the highest risk of miscarriage (about one in fifty pregnancies). It involves withdrawing blood from the umbilical cord and is primarily used when results are needed extremely quickly, or when only a fetal blood sample can provide a given answer about the fetus. For example, it may be used to test the fetus if the mother has been exposed to an infectious organism known to cause birth defects.

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