Congratulations on your pregnancy.
As Fiddle-Faddle's link said, people with DS are more likely to have celiac disease. I believe that it is recommended that they be screened on a regular basis reguardless of whether there is any family history. You didn't say it like this, but I have not heard of a link where someone with celiac disease is more likely to have a baby DS.
Was your ultrasound a high resolution ultrasound? If not, you may want to look into one. They can look for addtional DS markers (at mine, they measured the length of the pinky, the length of the thigh bone, the amount of fat on the back of the neck and a couple of other things). They can also look for a heart defect. I don't want to scare you, but a lot of DS kids are born with heart defects from minor to major. They might be able to give you a heads up on things to come or they might be able to tell you that they don't find any physical markers.
I would suggest that you make contact with a local DS support group. They are everywhere! And the people are wonderful!! One of the best things I ever did for myself was to go to a DS family picnic. There I watched and saw that these families were NORMAL families. Kids fighting over sharing! Mom's chasing after kids to get them to eat something! Loud Play! Mayhem! Everything you might expect when kids get together. It did my heart good.
Good Luck and Take Care of Yourself
I did have a level 2 ultrasound and they checked all the things you said above; he had no problems except what I said in my orginal post. Thanks for the idea about the DS support group.