This is always a tough one, since momma cat is feral...and to attempt to domesticate her at this point will be VERY tough, particularly since she may be extra defensive and protective as a new mother. But I absolutely agree that with the cold winter months becoming harsher, something needs to be done, as she and her kittens may not survive the dropping temps.
I would first look into renting a humane trap...this can often be done through local animal shelters or rescue organizations. Research everything online as far as trapping a feral, how to set the trap up, what to look for, when to check the trap, etc. Those that post on the Ferals and Rescue board on TCS will be an invaluable resource for these questions also.
If you're able to humanely trap her, bring her indoors at once, and keep her absolutely quarantined from any other household pets. Unfortunately, you may not be able to vet her or have her examined until you can establish some trust, and get her more comfortable in an indoor setting, if possible...this can take a while, and it takes a lot of dedication. I wouldn't even suggest removing her from the trap or enclosure for a while, for your own personal safety. In a situation like this, if she's visibly pregnant, she may deliver within a week or two...so make sure that you're at least in contact with a vet to help guide you with this, or in the event there's complications.
I guess that's the best advice I can give you. Rent a trap, find a way to trap her, bring her inside, contact a vet, and try to establish some trust. If she delivers and cares for her kittens successfully, it would be best for her to be spayed prior to her being rehomed, or placed back into her outdoor setting, so that she does not continue to breed.