Term	Definition
Combined screening	Combination of nuchal translucency testing, serum measurement of PAPP-A and free or total hCG, and maternal age
Contingency screening	Nuchal translucency, PAPP-A, and hCG measurements are used to determine risk; no further testing is recommended in women at low risk of Down syndrome; maternal serum AFP should be performed in the second trimester to assess risk of neural tube defects
hCG	Hormone made from the fetal part of the placenta; elevated levels associated with increased risk of Down syndrome
Inhibin A	Hormone produced by the placenta; high levels associated with increased risk of Down syndrome
Integrated screening	Nuchal translucency testing and measurement of PAPP-A and hCG are integrated with the quadruple screen to produce a single risk result
Maternal serum AFP	Plasma protein found in mother’s blood that is produced by the fetal liver; elevated levels associated with open neural tube defects, older than expected fetus (i.e., dating error), or multiple gestations; low levels associated with Down syndrome
Nuchal translucency	Thickness of the fluid under the skin of the fetal neck, measured by ultrasonography
PAPP-A	Zinc-binding protein that acts as an enzyme; low levels associated with increased risk of Down syndrome
Stepwise sequential screening	Nuchal translucency testing and measurement of PAPP-A and hCG are used to determine risk; if at increased risk, proceed directly to invasive testing; if not at increased risk, a second calculation with the addition of the quadruple screen produces another risk determination
Unconjugated estriol	Protein produced by the placenta and the fetal liver; low levels associated with Down syndrome