What are the chances of harming my baby if I fall on my stomach?

See below. Usually minimal. You naturally try and get your arms in front to take a fall. The child is also somewhat protected by the tissue surrounding it. I would suggest going to your OB doctor or an urgent care center if you fell directly on your abdomen to check for fetal heart tones.
<1 % Trauma to the uterus such as a direct fall or car accident, etc can rarely cause the placenta to separate. This is called an abruption. It is unlikely before the third trimester and requires a significant blow to the abdomen. The symptoms would be uterine contractions, pain, and/or bleeding.