First of all, congrats to your family on the new arrival!
I know how scary it is to find out something may be wrong with your baby, my son was diagnosed at birth with Down syndrome 3 years ago. If the baby does have Down syndrome, then your niece will most likely do a lot of grieving for the typical baby she had imagined welcoming to the world. One word of caution...most people really really don't want to be told, "I'm sorry." Personally, I didn't mind because *I* was sorry my son had Down syndrome, I certainly wouldn't have chosen that for him. But the majority of parents of kids with Down syndrome feel very strongly that they want to hear "Congratulations, what a beautiful baby", and they don't want sympathy cards, they want to celebrate their baby, even if the baby comes with a "little extra". Oh, and please don't send her "Welcome to Holland"--about a billion other people will, so she won't need to read it again.
One thing that is huge in the disability community in general, and for people with Down syndrome in particular, is to use people first language. I wasn't familiar with this before the birth of my son...but it is more than semantics and political correctness, our words really do shape the way we see the world. So please be aware that it isn't a "Down syndrome" baby or even worse a "Downs baby" or the worst...a "Downs". It is a baby with Down syndrome. Down syndrome is NOT the defining characteristic of the individual. It may seem like it at first as a family comes to terms with the diagnosis, but looking at my book-loving, soccer-ball kicking, funny, curious, Blues Clues fan, it is clear that Down syndrome is a very small part of who he is.
Secondly, while it is rare, I can think of at least 5 babies with Ds that I know personally who were not diagnosed immediately at birth. The facial features are not always evident even to professionals, and there are babies with Ds who do *not* have low tone. Also, not every baby with Ds will have cardiac problems. Some kids with Ds have no medical issues and are the healthiest kids in their families. Lukas is going to the dr. today for his 3 yr. check up for the first time in 6 months.
If the baby does have Down syndrome, there is tons of information and support out there. The blog alwayschaos probably has the best collection of links. They are on the right side when you scroll down.
I would highly recommend *not* reading the book Babies with Down syndrome, and can't speak well enough about the DVD Down syndrome: the first 18 months. It isn't cheap, but it is WONDERFUL, and really gives a new parent lots of hope.
To see a beautiful montage of babies and kids with Down syndrome:
Also, just to get a glimpse of what is possible for people with Down syndrome today, check out these beautiful pictures...people with Down syndrome are living independently, working, paying taxes, getting married, and living full and happy lives.
Please let us know the outcome, regardless of how the chromosomes come back I hope that the baby is healthy and getting the hang of eating!