When they do the initial blood test they are looking for markers. These only indicate whether there is a possibility of the baby being born with downs. In Canada (sorry no stats for elsewhere) only about 3% of women whose test comes back with markers go on to have a downs baby. If your results show the marker, they move onto the amnio check (the big needle) this test is not without dangers tho, can cause an infection, preterm labour or even a misscarriage. And as others have said, they will do some checks via ultrasound to look for abnormalities.