They've already done research on this. And most people indeed do choose to terminate pregnancy if they the test came back positive.
http://onlinelibrary.wiley.com/doi/10.1111/j.1542-2011.2011.00109.x/abstract
23%-33% of prospective parents said they would terminate
46%-86% of parents with increased risk would terminate
89%-97% did terminate when they received positive diagnosis.
What people say and do are two different things. I somewhat lean against not terminating myself but I don't blame people that would terminate.
The last figure also puts me in doubt that 40-50% of people are truly "pro-life".
If people know that they are not going to abort regardless of the results, they are less likely to test.
This exactly what my wife and I did during her pregnancy. I voted no on the poll and yet... the flesh is weak. We didn't want to deal with temptation to murder our child because they'd be disabled or a challenge to care for, so we opted out of the Down Syndrome screening.