Test reports for prenatal screening panels typically include a calculated risk factor based on the normal levels for the test or tests. The average normal for the test is termed the “population median” and test results are compared to this normal and reported out as Multiples of the Median (MoM). The average value for a normal median for a singleton gestation is 1.0 MoM. Hence, any prenatal test that is below average or decreased would be considered less than 1.0 MoM, whereas, a test that is above average or increased would be greater than 1.0 MoM.* The laboratory typically establishes a MoM cutoff value for each test, which classifies each screen as either screen-positive or screen-negative. In addition, important considerations for determination of the risk factor for Down syndrome are the age of the fetus (gestational age), whether this is a singleton or multiple gestation, maternal age, and maternal race. All results are typically entered into a special software program that calculates the total risk. Results are reported as “screen-negative” or “screen-positive” and provide an estimate of the risk of chromosome abnormality. However, it is important to note that serum screening tests cannot diagnose Down syndrome. To accurately diagnose Down syndrome, a fetal chromosome analysis must be performed using an invasive procedure, such as chorionic villus sampling (CVS) or amniocentesis testing.
* Twin pregnancies would require a different screening cutoff level. For example, some laboratories use a screening cutoff between 3.5 and 5.0 MoM for twin pregnancies. HCG and DIA levels are approximately twice as high in twin pregnancies and uE3 level is approximately 1.7 times as high.