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First Trimester
Down Syndrome Screen

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Formal name: First Trimester Screen [Pregnancy-associated plasma protein-A (PAPP-A), human chorionic gonadotropin (hCG) as either free beta subunit or total hCG, and nuchal translucency ultrasound]

The Test Sample

What is being tested?

The first trimester screen is a combination of two blood tests and a special ultrasound that are used to assess a pregnant woman's risk of carrying a baby with Down syndrome (trisomy 21) or Edwards syndrome (trisomy 18). Performing and evaluating them together, along with the woman's age, increases both the sensitivity and specificity of the screening results.

  • Pregnancy-associated plasma protein A (PAPP-A) is a protein produced first by the outer layer of the developing pregnancy (trophoblast) and then by the growing placenta. During a normal pregnancy, levels of this protein increase in the pregnant woman's blood until delivery.
  • Human chorionic gonadotropin (hCG) is a hormone produced by the developing pregnancy and then produced in large quantities by the placenta. Either free beta subunit or total hCG can be used in first trimester screening. Levels of both usually rise rapidly in the pregnant woman's 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 measurement made by ultrasound. The ultrasonagrapher measures the fluid collection between the spine and the skin at the nape of the fetus's neck. It is a procedure that requires a specially trained radiologist, proper alignment of the fetus, and careful measurement. It is not a routine ultrasound, and it is not a procedure that is available at every hospital or health facility.

If the results of first trimester screening are cause for concern, further screening tests (e.g., quad screen) or diagnostic tests (e.g., amniocentesis or chorionic villus sampling (CVS)) may be recommended.

How is the sample collected for testing?

Blood is drawn from a vein in the woman's arm or collected from a finger stick. The nuchal translucency ultrasound may be performed from outside the abdomen (transabdominally) or the probe may be inserted into the vagina (transvaginally).

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

You may be instructed to have a full bladder when having the nuchal translucency ultrasound performed.