You should not do it period. I mean sleep on stomach during pregnancy, even if you cannot fall asleep any other way, avoid this at all costs.
You may want to consider getting in more comfortable position with the use of the whole body pillow. I almost never slept on the stomach, but during my pregnancy I felt like I wanted to and it was against the rules as I could hurt the baby. So no, no no!
In addition, you need to have a healthy diet plan, as explain in this free guide because a good exercise and of course a great meal plan, will help you sleep better, and you will stop sleeping on stomach during pregnancy!