Amniocentesis is still the diagnostic test that gives a definitive answer in every case. However, the procedure also carries a risk of losing the fetus (1 in 200 to 1 in 300). A series of screening tests have been developed over the past few years that provide women aged 35 and over with an alternative to amniocentesis. The detection rate of these screening tests, however, is never 100%. You should therefore understand and accept the fact that, while these screening tests avoid amniocentesis and its risks, a number of chromosomal abnormalities may remain undetected, depending on the test selected. If you want to be 100% sure that the fetus has no chromosomal abnormality and you are 35 years of age or older, you should opt for amniocentesis. Talk to your doctor about which test is most likely to meet your requirements.
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These tests are certainly not compulsory. They are available to those who consider it important to have the information they can provide. It may be worthwhile for you to have such a test if it is important for you to know whether there is a risk of your unborn child carrying trisomy 21. You should also realize that, if the results are abnormal, the next step is amniocentesis in order to have a definite diagnosis.
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Nuchal transparency is a tool used to screen for trisomy 21. It is 70% accurate when used alone. It is important for the people measuring nuchal transparency to be experienced and specially trained, otherwise the accuracy level will be even lower. When combined with 1st and/or 2nd trimester blood markers, the test's accuracy improves. You should discuss the availability of nuchal transparency screening in your area with your doctor.
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We use the ultrasound expected delivery date when there is more than five days' difference between this and the one based on the date of the last menstrual period. When an ultrasound is done during the second trimester (at 20 weeks), the two dates must differ by more than 10 days for the ultrasound date to be used.
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