Call the humane society and ask their policy. If she's friendly and healthy, she'll be spayed and placed with an adoptive family (as will her kittens). If she's sick (i.e. has feline leukemia, for instance) then she'll be put down (which is MUCH more humane than letting a sick cat die of a disease out in the elements).
We fed a feral stray cat, she had kittens in the water drainage ditch, then suddenly we were feeding six. Three eventually disappeared (over the course of a year and a half) and I finally tamed mama and two kittens "enough" that I could finally grab them while they ate on the porch (one at a time over a period of a year or more). I took those three to the humane society b/c I couldn't bear having them face another winter outside. The year before we'd had a number of nights at 10 degrees (in Seattle!!) Anyway - each time I took a cat in to the humane society I'd donate whatever I could - between $100 and $125 bucks. Guilt money, I suppose. But i'm glad we don't have a bunch of strays mutiplying, starving and dying in our neighborhood.
As an aside, the cat food on our porch drew not only the cats, but dogs and raccoons, too. Raccoons can be vicious and BOLD - I didn't want an raccoon encounter with my own sweet cat or my husband or liitle boy, for that matter!