Will my cat come back?

I have a cat named Shadow. I went on a vacation and came back to find out my cat went missing. I know some cats disapear for a while then come back, but my cat has been gone for 3 weeks. Shadow is pregant with kittens, but she isn't due tell the beginning of July. My dad came home while I was still on vacation. When he came home, he saw her for 2 days then she was gone. She hasn't came back home to eat or anything. I am getting worried. I live on a farm 30 min. out of town where a bunch of rattle snakes, and coyotes live. I am affraid that she ran somewhere looking for me and got lost. I called for her everywhere except out in the fields. If she herd me she would come back. If she is still alive, will there be a chance she will come back? Its been for 3 weeks since she hasn't been here with no food or water. Will she live? Will she come back? How can I find her? I just miss her sooo much, I wish I could see her again. Please try to help. Thank-you.