I''m worried about having a Down''s baby...there are so many tests available. What is the difference between a screening and a diagnostic test?

A screening test for Down''s is one that gives a risk level of having a Down''s baby. For example a ''1 in 20'' risk means that if 20 women have exactly the same result as you, 1 will have a baby with Down''s and 19 won''t. Similarly, ''1 in 2000'' means 1 baby will have Down''s, 1999 won''t. Common screening tests for Down''s include a blood test at 15-17 weeks (triple test/serum screening/Bart''s test), or ultrasound ''nuchal'' scan at 11-14 weeks. A diagnostic test gives a definite result (''no the baby doesn''t have Down''s'' or ''yes the baby has Down''s) but requires an ''invasive'' (needle) procedure such as amniocentesis or chorion villus sampling. The downside of an invasive test is the risk of miscarriage (about 1%), but the downside of a screening test is that it doesn''t give a definite answer.
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