Towards the end of your first trimester, usually between weeks 11 and 14.
They can gauge your risk of having a baby with Down syndrome or other chromosomal disorders. A screening test will show whether there is an increased risk for a chromosomal abnormality, but only a diagnostic test will give you definitive answers.
Screening tests. You'll be offered a blood test that measures PAPP-A (pregnancy-associated plasma protein-A) and free beta hCG (human chorionic gonadotropin), along with an ultrasound to measure the skin thickness on the back of the baby's neck (nuchal translucency). These measurements are then incorporated into a complicated formula that includes your age; the result tells you the likelihood that your baby has a chromosomal abnormality. This screening detects more than 80 percent of Down syndrome cases. The screening test will suggest a problem approximately three to five percent of the time—but you'll need additional tests to verify the diagnosis.
Diagnostic tests. If your screening test suggests an increased risk for chromosomal abnormality, further testing, such as a chorionic villus sampling (CVS) or amniocentesis, will help you get a definitive answer. (Don't worry—even if the screening test is positive, in most cases, the diagnostic tests will show that the fetus has normal chromosomes. These first trimester screening tests are not available everywhere, and the technique used to measure the tiny fold on the back of the baby's neck must be done by an experienced sonographer or you may get misleading information. If first trimester screening is not available where you live, don't despair. Second trimester screening blood tests, which are universally available, work very well and have a comparable detection rate.
Originally published in You & Your Baby: Pregnancy.
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