> A valid and reliable survey instrument was mailed to 4,924 households on the mailing lists of six non-profit Down syndrome organizations.

Definitely no sampling bias here... And given that the vast majority of people who do prenatal screening decide not to have a child with Down Syndrome, I don't think the people who choose to have a Down Syndrome child are really representative of prospective parents as a whole.

The revealed preference is clear, particularly in places like Iceland where prenatal screening is ubiquitous. They have effectively eradicated Down Syndrome going forward.