The Genzyme test is what some other doctors are calling the first trimester screen. It is a combination screening that uses both the maternal blood test previousy done around 16-20 weeks gestation (often called the triple scrren, quad screen, AFP, or alpha-fetal protein test) and an ultrasound to measure primarily the thickness of the nuchal fold (skin on the back of the babies neck). This combined approach has made the testing much more accurate than the bood test alone, but there still are a number of false positives. If you do test positive it is not a definitive diagnosis for Down's Syndrome, they will give you the results in an odds ratio (i.e. you have a 1 in 20 chance of having a bay with Down's Syndrome). They will recommend an amnio to know for sure, but that will still be up to you, you can refuse further testing.
Many people, as indicated by previous responses, believe there is no point to having the test if you would not abort the pregnancy based on the results. But I think parents can benefit alot by being aware of the confirmed or suspected diagnosis before the birth. Knowing the sex of a baby before it is born is important to so many parents, not because they will abort one sex or the other, but so they can prepare (i.e. names, clothes, nursery theme, etc). Preparing for a special needs baby (getting educated on the diagnosis, grieving for the loss of a "perfect child", teaching siblings/family/friends, etc) during the pregnancy rather than after the birth can make for an easier transition. Things can already be trying enough postpartum without the added stress of an unexpected diagnosis.
Good luck in whatever decision you make.
S.