pregnant women have access to both screening and diagnostic tests to find out if an unborn child has Down syndrome. screening tests estimate the risk of the fetus having Down syndrome, whereas diagnostic tests tell whether or not the fetus actually has the condition.
screening tests are typically offered between 8 and 20 weeks of gestation, but they can accurately detect about 60 percent of cases. Many women who undergo these tests will be given false-positive readings, and some women will be given false-negative readings. prenatal diagnosis of Down syndrome is by chorionic villus sampling (CVs), amniocentesis, and percutaneous umbilical blood sampling (PUBS).
Each of these procedures carries a small risk of miscarriage as tissue is extracted from the placenta or the umbilical cord to examine the fetus's chromosomes, but the procedures are about 98 to 99 percent accurate in the detection of Down syndrome. Amniocentesis is usually performed between 12 and 20 weeks of gestation, CVS between eight and 12 weeks, and PUBS after 20 weeks.
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