Your baby is pretty well protected in your womb from any type of harm when you're sleeping or moving around.
However if you're worried about hurting the baby in the womb then you could sleep on your back if possible and then no harm to the baby would be done.
However I've slept on my side while pregnant and the baby was perfectly fine but I would avoid sleeping on your stomach just in case that happens to hurt the baby.
Your rib cage mostly protects the baby in the womb from any harm when sleeping so I wouldn't worry about it but I would not sleep on my stomach.
However sleeping on your side or back will do no harm to your baby.