Usual Down's syndrome testing takes place at sixteen to eighteen weeks, during the second trimester of pregnancy. By this time, an abortion which involves labor and delivering the fetus. For most women, this is not a viable option.
The standard test is a blood diagnostic test, nick-named the 'triple-test'. The test measures the levels of three proteins. Beta-hCG, is one of the tested proteins. Beta-hCG is part of the human chorionic gonadotrophin hormone, also the protein detected by most pregnancy tests. This protein is seen in elevated amounts in women carrying fetus' with Downs syndrome. The other two tested proteins are estriol and alpha fetoprotein. Estriol is found in reduced amounts in the Downs pregnancy. Alpha fetoprotein is also found in reduced amounts in Downs, but elevated in cases of neural-tube defects, such as spina bifida. This test detects about sixty to seventy percent of fetus' with Downs.
Women with positive test results for the triple-test are encouraged to have amniocentesis testing. This test involves the injection of a long needle to collect samples of amniotic fluid. The cells in the sample are then broken open and examined for the extra chromosome that caused Downs. This test does have it's risks. Amniocentesis may result in infection or leaking of amniotic fluid. Leakage may cause the development of a clubfoot, believed to be the result of the decrease in development space for the fetus.
The new testing has been able to identify fetus' as early as ten weeks with Downs, without the use of amniocentesis testing. The new method combines ultrasound and blood tests. The blood tests used detect beta-hCG and pregnancy-associated plasma protein A (PAPP-A). Only recently, scientists have found that PAPP-A resides in reduced amounts in women carrying fetus' with Downs. Researchers then use ultrasound to examine the thickness of the back of the neck in the developing infant. It has been found that the increased thickness of the nuchal (neck) membranes indicates Downs syndrome.
By the end of 2000, results of a test involving six thousand pregnant women in the United States will be released.
Thank you to Scientific American for the source of my information
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