Genetics

Prenatal Screening and Testing

Genetic testing is used to determine whether a fetus has a disease or genetic abnormality such as Down syndrome or trisomy 18. Information obtained from this type of testing may help guide decisions during pregnancy.

Other tests are used for prenatal screening to determine whether a fetus has an increased risk for a genetic disease. These tests can help identify substances that might indicate a genetic disease. Examples of tests used for prenatal screening include:

• Triple screen test. This test measures levels of three substances in a pregnant woman's blood that indicate how likely it is that the fetus has Down syndrome, trisomy 18, or a neural tube defect.² The results of this test are combined with other information (pregnant woman's age, weight, race, and whether she has diabetes) to estimate the risk of Down syndrome in the fetus. Sometimes the tests indicate that a fetus is likely to have Down syndrome but the fetus is unaffected. This is called a false-positive result. Or sometimes screening tests do not find the condition when it is present. This is called a false-negative result. The substances measured in the triple screen test are:
  • Alpha-fetoprotein (AFP), a substance naturally produced by the liver of a fetus.
  • Human chorionic gonadotropin (hCG), a hormone produced by the placenta during pregnancy.
  • Estriol (uE3), a hormone produced by the placenta during pregnancy.
• Quad screen test. This adds another hormone (inhibin A) to the three substances tested for in the triple screen. The results of this test are combined with the pregnant woman's age, weight, race, and whether or not she has diabetes to help determine whether the fetus has a greater chance of having a condition such as trisomy 18, Down syndrome, or a neural tube defect. The results of a quad screen are generally more accurate than the results of a triple screen.
• Fetal ultrasound. This test uses reflected sound waves to produce a picture of the fetus, the placenta, and amniotic fluid. It is used to determine whether the fetus has a structural abnormality, such as a heart defect.

In some cases a combination of screening tests is done in the first trimester to look for Down syndrome or trisomy 18. This is sometimes called integrated screening. The screening test uses an ultrasound measurement of the thickness of the fetus's neck (nuchal fold or translucency) and the levels of hCG and a protein called pregnancy-associated plasma protein A (PAPP-A). The sensitivity of this screening test is about the same as that of the second-trimester maternal serum quad screening.³

If prenatal screening test results are abnormal, further genetic testing (karyotype) can be used to examine the size, shape, and number of chromosomes. A karyotype can be done on cells taken from the placenta (chorionic villus sampling) in the late first trimester or from the amniotic fluid (amniocentesis) in the second trimester. Extra, missing, or abnormal positions of chromosome pieces can cause problems with growth, development, and body functions.

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Author:	Maria G. Essig, MS, ELS	Last Updated: November 20, 2007
Medical Review:	Anne C. Poinier, MD - Internal Medicine Siobhan M. Dolan, MD, MPH - Reproductive Genetics
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