It is very important that you keep her 100% inside until she has given birth, her kittens have weaned, and she can be spayed. Moms can get pregnant again within a week of delivery, and without going into heat. In addition, moms that go out often bring back illness or parasites to their kittens, or don't come back at all.
Now is the time to stock up on kitten supplies! You should get a small scale to weigh them daily, powdered kmr (PetAg is great), and a needleless syringe in case you need to supplement.