Triple/Quad Screen: Test Sample

What is being tested?
The triple screen is a group of three tests that are used to screen pregnant women in the second trimester of pregnancy. The quad screen adds a fourth test to the group. Each test that is performed measures a different substance found in the blood: AFP, hCG, unconjugated estriol, and with the quad – inhibin A. The newest marker, inhibin A, increases both the sensitivity and specificity of the screen.

Alpha-fetoprotein (AFP) is a protein produced by fetal tissue. During development, AFP levels in fetal blood and amniotic fluid rise until about 12 weeks, and then levels gradually fall until birth. Some AFP crosses the placenta and appears in the maternal blood.
Human chorionic gonadotropin (hCG) is a hormone produced by the placenta. Levels in maternal blood rise for the first trimester of pregnancy and then fall to less than 10% by the end of pregnancy.
Unconjugated estriol (uE3) is a form of estrogen that is produced by the fetus through metabolism. This process involves the liver, adrenals, and the placenta. Some of the unconjugated estriol crosses the placenta and can be measured in the mother’s blood. Levels rise around the 8th week and continue to increase until shortly before delivery.
Inhibin A is a hormone produced by the placenta. Inhibin is a dimer (has 2 parts) and is sometimes referred to as DIA or dimeric inhibin A. Levels in maternal blood decrease slightly from 14 to 17 weeks gestation and then rise again.

In pregnancies where the fetus is carrying the chromosomal defect that results in Down syndrome (trisomy 21), the levels of AFP and unconjugated estriol tend to be low and hCG and inhibin A levels high.

In pregnancies where the fetus has trisomy 18, unconjugated estriol and hCG levels are low and AFP levels can be variable.

A baby with an open neural tube defect has an opening in its spine, head, or abdominal wall that allows higher-than-usual amounts of AFP to pass into the mother’s blood. The other markers are not used in the evaluation of risk for carrying a fetus with a neural tube defect.