During the beginning of pregnancy the uterus is completely protected by the pelvis so by pushing on your stomach really hard or falling on your stomach you should not cause any harm to the baby.
During this time the uterus isn't exposed so the baby cannot be harmed.
During the 18 to 20 weeks of pregnancy the baby can usually feel you poking or pushing on your stomach and may even kick back or poke back which you can then feel.
Sometimes your baby will even kick you inside your stomach which is funny but not harmful to you or your baby.
My son kicked me in my belly when I was pregnant with him when I was poking around on my belly and I got a reaction from him.
Sure wish I could've actually seen him doing that but couldn't.
Still it's funny.