No reason for a cat to give birth or raise kittens outside if the cat feels safe at home. What pet owner would be ok with their cat giving birth outside and raising her kittens outside anyways?
If you want to get close to the mom cat and kittens there is an easy way. Get a bag of wiska temptations, it is like cat cocaine, the kittens and mom cat will stick around so long as you keep feeding them the cat treats.