By doing an ultrasound the sonographer measures the NT ( NEURAL TUBE THICKNESS) and the presence or absence of the nasal bone plus the flow of blood through the Ductous venoses and few other soft markers.. Typically a baby with Down's syndrome has an increased NT and absent Nasal bone or a hypoplastic nasal bone... The Double marker test measures- BhCG and PAPP-A which along with the USG helps to predict the possibility of having an abnormal baby... When the combined screen test is positive.A DIAGNOSTIC METHOD IS ADVISED eg- AMNIOCENTESIS OR CVS.whereby the fluid around the baby or few cells near the cord insertion are taken for examination under the microscope and then the genetic component (chromosomes) in the cell are studied to determine whether the child born would be normal or affected.