Whoa, be careful!!!! If she's truly feral, then you must avoid eye contact with her, and do not approach her...you could get seriously injured. If she's a stray, that's a different story...that means she has had enough interactions with people to be a "people cat" or she was someone's previously owned cat. Please do not expose your other animals to this female until you get her to the vet immediately...you must be aware of her health status before you can proceed to do anything. And, since she is pregnant, she must absolutely remain indoors at all times now, until the kittens are weaned at 10-12 weeks.
Again, no interaction with other cats in your home until you get the "all clear" from your vet. You just never know. Is it also possible that this momma has other kittens stashed around outside who need her to care for them?
Hmm...lot's of questions to think about here.