These tests test alpha-Fetoprotein (AFP) levels, chorionic villus sampling, and Amniocentesis. In the first trimester, a blood test and ultrasound are done to measure the size of the clear space in the tissue at the back of a baby's neck. During your second trimester, another blood test is called the quad screen. This test measures levels of four substances in your blood. Prenatal cell-free DNA screening provides information about a baby's sex and rhesus (Rh) blood type. These tests detect chromosomal aberrations and down syndrome identification.