The OB told me as long as I was physically comfortable I could have sex. Your body was designed to protect the baby. I had sex through all four of my other pregnancies and they turned out just fine =)
I am pregnant now with twins and had a cervical cerclage so no sex for me this time until after birth =( ... which *****! Have to do what I have to do though ya know?
Good luck!
You can have sex however you're comfortable and in whatever position your comfortable in. It is safe to be on your belly, the baby has plenty of padding in there. Having sex won't hurt your baby.
You can have sex. I suggest just dont lie on your tummy.