Join Date: Aug 2013
Location: San Francisco Bay Area.
I've taken in pregnant strays twice, kept them, had the kittens and found home for the kittens and it was an awesome experience.
So, look forward to a very enjoyable experience. You have the Mom now, and maybe a month until the babies come. That's great, you have time to really build trust and a bond with the Mom kitty. It is important to feed the Mom well through her pregnancy.
There are many threads here about the quality of food and it's cost. I think another reply suggested you might contact local rescue or other groups and ask for help.
My theory is adding cooked chicken, without skin or bones and finely chopped enriches their diet and is inexpensive.
Their is much info on the web about what to expect and how to help. Feed her well, spend time with her and build trust, set up a quiet hidden spot for her to have her babies. She may want you with her, or not.
Mostly, support her and enjoy along with her. It's fun and heartwarming.