Down syndrome diagnostic tests done during pregnancy include:
Amniocentesis, which takes a sample of amniotic fluid, the fluid that surrounds your unborn baby. It is usually done between the 15th and 20th week of pregnancy.
Chorionic villus sampling (CVS), which takes a sample from the placenta, the organ that nourishes your unborn baby in your uterus. It's usually done between the 10th and 13th week of pregnancy.
Percutaneous umbilical blood sampling (PUBS), which takes a blood sample from the umbilical cord. PUBS gives the most accurate diagnosis of Down syndrome during pregnancy, but it can't be done until late in pregnancy, between the 18th and 22nd week
They tell by looking at the baby’s chromosomes.