Are there fewer and fewer cases of down syndrome now that we have prenatal testing?

No: But many of those cases are detected by the currently available testing, and some women elect to terminate their abnormal pregnancies. So there may be fewer abies born with down's syndrome.

Pattern change: After the introduction of the Pill, more older women are able to avoid pregnancy,so the issue comes up less often in that population.Many younger women elect not to have prenatal testing and although they have less risks/pregnancy, a baseline of predictable DS births will remain.