AFP and trisomy 21: Other studies show that effective first-trimester screening for trisomy 21, with a detection rate of 98% and invasive testing rate < 0.5%, can be potentially achieved by contingent screening incorporating NIPS and NT, ductus venosus pulsatility index for veins, serum-free β-hCG, PAPP-A, placental growth factor and alpha-fetoprotein [18].