If this is of concern, I would definitely recommend reading Dr Sarah Buckley's book Gentle Birth, Gentle Mothering, no matter what parenting orientation you take.
It has a couple of really good discussions about prenatal diagnostic testing, with much reference to research about not only incidence of DS and other congenital 'abnormalities', but also quality of life studies of the mothers of these kids. It's an awesome read that serves to really get you at peace with whatever's going on If your local library doesn't have it, you can request it, and your local ABA group might even have it in their library.