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 Related tests:Triple/Quad Screen, total hCG
What is being tested? The first trimester screen is a combination of two blood tests and a special ultrasound that are used to screen pregnant women in the first trimester of pregnancy. Each test measures a different factor that is altered in a fetus that has chromosomal abnormalities such as Down syndrome or trisomy 18. Performing and evaluating them together increases both the sensitivity and specificity of the screening results.
PAPP-A is a protein produced first by the trophoblast (outer layer of the developing pregnancy) and then by the growing placenta. During a normal pregnancy, levels of this protein increase in the mother’s blood until delivery.
hCG is a hormone produced by the trophoblast and then created in large quantities by the placenta. Either free or total hCG can be used in First Trimester Screening. Concentrations of both usually rise rapidly in the mother’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 an ultrasound that measures the space (thickness) 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.
How is the sample collected for testing? Blood is drawn from a vein in the arm. The nuchal translucency ultrasound may be performed transabdominally (from outside the abdomen) or transvaginally (the probe is inserted into the vagina).
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.
This article was last reviewed on July 5, 2007.
This page was last modified on April 8, 2009.
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