During a maternal serum screening the mother's blood is checked for a combination of different markers: alpha-fetoprotein (AFP), unconjugated estriol (uE3), and human chorionic gonadotropin (hCG) make up the standard tests, known together as the "triple test." Sometimes a marker called inhibin A is added, making the "quadruple screen." These tests are independent measurements, and when taken along with the maternal age (discussed below), can calculate the risk of having a baby with Down syndrome. Over the last fifteen years, these were done in the 15th to 18th week of pregnancy. Recently, another marker called PAPP-A was found to be of use even earlier.
A very important consideration in the screening test is the age of the fetus (gestational age). The correct analysis of the different components depends on knowing the gestational age precisely. The best way to determine that is by ultrasound.
Once the blood test results are determined, a risk factor is calculated based on the "normal" blood tests for the testing laboratory. The average of normals is called the "population median." Test results are sometimes reported to doctors as "Multiples of the Median (MoM)." The "average" value is therefore called 1.0 MoM. Down syndrome pregnancies have lower levels of AFP and estriol, so their levels would be below the average, and therefore less than 1.0 MOM. Likewise, hCG in a Down syndrome pregnancy would be greater than 1.0 MoM. In the serum screening, the lab reports all results in either this way or as a total risk factor calculated by a software program.