Maternal Serum Marker Screening Test

What is a maternal serum marker screening test?

Given between 15 and 20 weeks of pregnancy, this blood test checks for birth defects such as Down Syndrome, Trisomy 18, or open neural tube defects.

How is the test performed?

Your doctor will take a sample of your blood and test it for three chimicals: lpha-fetoprotein (AFP) (made by the liver of the fetus), and two pregnancy hormones: estriol and human chorionic gonadotropin (hCG). Sometimes, doctors test for a fourth substance in the blood called inhibin-A. Testing for inhibin-A may improve the ability to detect fetuses with a high risk of Down syndrome.

What’s Detected and the Results

igher levels of AFP are linked with open neural tube defects. In women age 35 and over, this test finds about 80% of fetuses with Down syndrome, trisomy 18, or an open neural tube defect. In this age group, there is a false positive rate (having a positive result without actually having a fetus with one of these health problems) of 22%. In women under age 35, this test finds about 65% of fetuses with Down syndrome, and there is a false positive rate of about 5%.