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First Trimester Prenatal Interpretive Screening

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The first trimester screen is a combination of two blood tests (PAPP-A, free-hCG) and a special ultrasound (Nuchal Translucency) that are used to screen pregnant women in the first trimester of pregnancy. This test is used to assess the fetus a pregnant woman is carrying for the risk of a chromosomal abnormality such as Down syndrome (trisomy 21) or Edwards syndrome (trisomy 18).  Performing and evaluating PAPP-A and free-hCG together increases both the sensitivity and specificity of the screening results.

PAPP-A is a protein produced first by the trophoblast and then by the growing placenta. During a normal pregnancy, levels of this protein increase in the mothers’ blood until delivery. hCG is a hormone produced by the trophoblast and then created in large quantities by the placenta. Concentrations of both usually rise rapidly in the mothers’ circulation for the first 8 to 10 weeks, then decrease and stabilize at a lower level for the remainder of the pregnancy. Nuchal translucency is an ultrasound that measures the space (thickness) between the spine and the skin at the nape of the fetus neck. A mathematical calculation using the results obtained from the PAPP-A, hCG, and nuchal translucency ultrasound is used to determine a numeric risk of a chromosomal defect in the fetus.  The first trimester screen does not, however, assess risk of neural tube defects, such as spina bifida, which can be done with the triple screen (women in the 15th to 21st week of pregnancy).

 

When is it ordered?

The test is usually ordered between 11 and 14 weeks of pregnancy.

 



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