The first trimester screen is a combination of two blood tests (PAPP-A, hCG) and a special ultrasound (Nuchal Translucency) that are used to screen pregnant women in the first trimester of pregnancy. It is perfomed at 11-13 weeks of gestation. 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 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. Either free or total hCG can be used in first trimester screening. 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 fetuss 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. This risk is compared with an established cut-off. If the risk is higher than the cut-off value, then it is considered positive or increased. In pregnancies where the fetus is carrying a chromosomal defect, such as the extra chromosome material that results in Down syndrome (trisomy 21) or trisomy 18, the levels of PAPP-A tend to be decreased, the levels of hCG are significantly increased, and the space at the fetuss neck is larger than normal. See also: Pregnancy Associated Plasma Protein-A, Serum and Free beta-human Chorionic Gonadotropin (B-hCG), Serum.