PAPPA and trisomy 21: According to the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) guidelines, the risk for Trisomy 21 (T21) in twin pregnancies can be estimated in the first trimester using the combined test (nuchal translucency (NT) combined with beta-human chorionic gonadotropin level and pregnancy-associated plasma protein-A level); as an alternative, NT and maternal age only can be used, but with an increase in false positive results [4].