I agree with Cmhjrc. They will more than likely offer you a CVS or an Amnio for further testing. My neice was born with a rare brain deformation that wasnt picked up on any sonograms or blood work. As soon as she found out she was pregnant again (had the IUD) they gave her so many options to make sure this baby was healthy. She had a baby boy in December. Good Luck.
I think Downs syndrome is chromosomal, not genetic. If you are over 35 years old your chances are increased however at any age you can have a child with downs. Just because you have one chil with downs, doesn't mean you'll have another. You should talk with your doctor or genetic counselor if you are concerned. No one on here is a doctor :)