While pregnant when is it unsafe to lay on your stomach?

You will feel it: As the baby grows and enlarges the uterus, this can press on the vena cava (large vein returning blood from your legs to you heart), thus reduces this flow. Laying on your back causes this problem and you will feel it--uncomfortable, and better when you are on either side. So, if you worry about it, then lay/sleep on your side to rests/sleep which you will need it. Congrats and good luck.