Maternal serum screening is a maternal blood test utilized during pregnancy to determine the risk of neural tube defects and chromosomal abnormalities such as trisomy 13, trisomy 18, and trisomy 21 (Down syndrome). The maternal serum screening can be performed in the first or second trimester. Maternal serum screening is a non-invasive prenatal test that assesses the risk of a condition. It is not a diagnostic test. 
Maternal serum screening
You can find the references annotated in each definition above at this link.