No pregnant woman is being OFFERED the opportunity to end her pregnancy based on the non-foolproof screening test, so no women are "choosing" not to end their pregnancies on that basis. Some women choose not to do further testing to find out of they are actually carrying a Down Syndrome fetus, but that's hardly the same as choosing not to end the pregnancy. Some women DO have the further testing, DO confirm that they are carrying a Down Syndrome fetus, and still choose to continue the pregnancy.
I was. It happens.