I just wanted to add that my pregnant stray (now our forever furball) showed up on our doorstep with a mild problem with fleas. I have no idea how ling she was outdoors, but I did notice some scratching and saw a few fleas. I had the same concerns as you.
My vet thought that since the flea problem wasn't severe, it would be better to keep changing her bedding often (washing everything in HOT water), comb her for fleas often and to NOT use any medications until the kittens were weaned, unless it became a serious infestation. I thought it would just get horrible in the weeks I had to wait.
It really didn't. I have no idea why. She had the kittens and every now and again, I would comb them all for fleas and only found a few here and there. I finally did treat them all when they were old enough and have not had any problems since.
I actually wonder if her constantly cleaning the kittens actually killed them off fast enough for it not to become a real problem, I don't know...just a theory.
Regardless, it's best to ask your vet for advice in your particular situation. Your vet may have a different approach. Just wanted to share my experience.