Between your 16th and 18th weeks of pregnancy, your health care provider will recommend screening blood tests that might detect whether your unborn baby is at increased risk for certain genetic abnormalities. These blood tests, either the triple screen or the slightly more accurate quadruple screen, check for abnormal levels of certain substances in your blood. Using those results, along with other factors such as your age, your provider will be able to asses the risk of your baby having a neural tube defect (NTD), Down syndrome, or other chromosomal defects.
These screening blood tests are offered to every pregnant woman. In contrast to the more invasive, diagnostic tests like amniocentesis or CVS, blood tests carry no risk to the pregnancy and they help identify pregnancies with increased risks. However, unlike those diagnostic tests, blood tests are non-invasive and carry no risk, whereas other more invasive, diagnostic tests, like an amniocentesis or a Chorionic Villus Sampling (CVS) in some rare cases lead to miscarriage. There's no reason to have every patient undergo an invasive test if it's not necessary -- so providers recommend additional testing only to those patients whose quadruple screen results indicate that their might be a genetic issue. In other words, these tests do not offer "positive" or "negative," "yes" or "no" results, but increased risk predictions. In fact, the tests sometimes give a false-positive result.
Along with the results from the quadruple screening, an ultrasound can aid your health care provider in determining whether additional testing is necessary. A sonographer can measure certain body parts on your baby-to-be to look for potential concerns. For example, increased thickness in the neck, a shorter-than-average nose length, and other physical clues might back up the high-risk results from the quadruple screen.Terms to Know
Triple screen: A prenatal blood screening that tests for the levels of certain substances in the bloodstream that might indicate genetic abnormalities in the fetus. In particular, the test looks at levels of alpha-fetoprotein (AFP), a protein produced by the fetus's liver and two hormones: human chorionic gonadotropin (hCG) and estriol. Also called triple marker or second trimester screening test.
Quadruple screen: A prenatal blood screening that tests for the levels of certain substances in the bloodstream that might indicate genetic abnormalities in the fetus. In addition to detecting levels of AFP, hCG, and estriol, as with the triple screen, the quadruple screen also tests for another hormone, inhibin-A. The quadruple screen is sometimes referred to as the quad test or the second trimester screening test.
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Images courtesy of the American Institute of Ultrasound in Medicine (AIUM.org).