Have Cat Questions? Ask a Cat Vet Online.
The labels on all of the prescription flea meds that you can get from your vet state that they cannot be used on pregnant animals but you might want to talk with your vet and see if it's worth the risk. You definitely want to get rid of the flea problem BEFORE the kittens get here because fleas are particularly dangerous for small kittens.
Until you can see the vet, I suggest that you treat the cat's bedding with flea prevention and then use a flea comb to remove all the fleas. When you are done, you can rub her down with a downy sheet or bounce. This is a natural and safe way to repel fleas for a short time and it makes the cat smell nice.
You CAN use a gentle flea shampoo if you want to take that route.
Let me know if you still have questions.