I also agree with K & L. The best thing to do is trap mama now and have her spayed, providing she's not that far along. If she IS about to pop, then maybe you can foster her and the kittens till their weaned and find homes. Everyone is looking for kittens these days.

If you let her stay outside while pregnant, she could have the babies and you may never find them, which would put more unspayed/unneutered cats out to multiply.

Good luck and please keep us posted!