The Double Marker (First Trimester Screening) test is a prenatal screening that measures the levels of two substances in a pregnant woman’s blood: pregnancy-associated plasma protein-A (PAPP-A) and free beta-human chorionic gonadotropin (β-hCG). This test, typically performed between the 11th and 14th weeks of pregnancy, helps assess the risk of certain conditions like Down syndrome (trisomy 21) and trisomy 18. It is usually combined with an ultrasound to provide a more accurate risk assessment for chromosomal abnormalities.