Oh this is so sad! Thank you for wanting to help this poor kitty.
Are there any rescue groups in your area? I'd do some searching to see if you can find a group that can help - maybe someone to foster her, so that she can be spayed after she has the kittens and so the kittens can be adopted out. All your neighbors need to know is that she disappeared for a little while?
If you live in the US, you can just type your zip code in the upper left hand corner to pull up organizations that may be able to help: http://www.pets911.com/organizations/organizations.php
You can also try contacting people here: http://www.alleycat.org/orgs.html
And by looking up animals to adopt, you can sometimes find other organizations (epecially fostering organizations) at these sites: http://www.1-800-save-a-pet.com/
(By seeing which orgs have animals to adopt, you can see who's involved in rescue!).
And this organization has an address you can e-mail to see if there's anyone in your area that can help: scroll down the page to find the e-mail address where you can write to ask for help: http://www.bestfriends.org/nomorehom...ets/about2.cfm
You can also contact Alley Cat Allies to see if there's a Feral Friend in their network near you: http://www.alleycat.org/feral_friends.html
I hope you find an organization or someone that can help.