First Welcome to PT Ally Cat's Mommy! It's wonderful to hear you'll help this kitty and her kittens to come. I agree it would be be best to trap her now. Try and find a place where the surroundings are as quit as possible and when you have trapped her, put a blanket over, it will help her relax. It will be easier to trap her if you let her go hungry first. Make sure there's a clean towel or blanket in the cage, if she gives birth when caught.

All the best of luck! Please keep us updated when you can. Be safe! Thanks for doing this!