There are a couple of good threads to read in the YT library about deciding whether or not to breed:
Breeding and Breed-Specific Library - YorkieTalk.com Forums - Yorkshire Terrier Community
I am not a breeder, but I have read many of the comments from breeder/exhibitors here and many veterans agree that they do not enjoy the whelping process because they worry so much about their girls.
It doesn't sound like your girl is going to reach 5 pounds. But, even if she were 8 pounds, you do have to seriously consider the possibility that you could lose her in the process.