I'm 10 weeks pregnant and fell on my stomach, will the baby be ok?

Usually OK: The bag of waters provides a great cushioning effect to most simple belly trauma that avoids a direct effect on baby. You should monitor yourself for spotting that might suggest the placenta was injured. If you have any vague or worrisome symptoms you should call your ob's answering service this weekend & seek direct advise.

Yes: Until 12 weeks your uterus is protected within your pelvic bones, so unless you broke your pelvis, it's unlikely that you'll cause harm. However, since a large number of pregnancies miscarry in the first trimester due to other reasons, you may want to seek reassurance from your ob.