Back or tummy?

My son was born at 37 weeks. They said he was border line preemie. Weird way to put it but whatever, I didn't come up with it. He had fluid in his lungs so he wasn't breathing regularly and they had him on his stomach the whole time he was in the NICU. It was only 3 days, but still. I never thought about it until you brought it up. My son is 2 now, so this was a while ago. So I have no idea why they put them on their stomachs.