I agree with Bina. Contacting a rescue group specializing in feral cats (TNR) is the best way to go. They can lend you the trap and will possibly have an experienced foster home available where she can have her babies. She should not be getting any vaccinations if she's pregnant.
I don't recommend trying this yourself as feral cats are a whole other ball of wax to deal with. They are basically a wild animal. At the very least, you will need a completely cat proof and escape proof room (separated from your other cats) where she can't injure herself trying to get out (and trust me, if she is truly feral she will try with every ounce of her being).
Regardless of what you decide to do, I thank you for looking out for her.
"To close your eyes will not ease another's pain." ~ Chinese Proverb
“We must not refuse to see with our eyes what they must endure with their bodies.” ~ Gretchen Wyler