Gestation period for a cat is 62-64 days. If you can bring her in and let her have her babies in a spare room, you'd be doing her a great favor. This way she'll be safe and warm. And as long as she lets you pet her, chances are she's not feral which will make it easier to place the kittens.

If you do bring her in, a birthing box is necessary. A cardboard box lined with plastic, old towels. Cut the edge of the box so that it's deep enough so the kittens won't climb out, but the mama can when she needs a break from her babies.

She'll need a quiet room with her own litterbox, food and water. I hope you'll do that for her as it's way too cold for her to have babies outside. They'll die.

Good luck and please let us know.