Koxka,

CrazyCatLady is right, she needs to be trapped now BEFORE the kittens are born. Even if you keep her in a cage in another room separate from your cats. She'll be safer, her kittens can be socialized very easily once they're born and weaned (before they turn 6 weeks old) and you won't have more unneutered/unspayed cats running around. If she isn't caught, she will move her babies if she feels they're in danger. Then you'll never find them.

Good luck!