I found this on Yahoo! LOVE it...for obvious reasons....
"The recommended weight is 5 pounds. I breed a 4 1/2 pound with no problems, but would never attempt to breed a 3 1/2 pound and I've been breeding Yorkies for over 20 years. Those that breed 3 pounders are in it for the money. They are willing to risk losing the dog because the puppies can bring a high price. Most likely, she will need a C-section which will be at least $1,000 and maybe more at an emergency clinic. If you are going to breed her, make sure you are prepared for the grief of losing her and/or the puppies. I really find it hard to believe that a vet would encourage breeding of such a small dog. The mate (and his lineage) should always be smaller than the female. YorkieTalk.com - Yorkshire Terrier Community is a very informative site just for Yorkie owners. It is full of stories of dogs and/or puppies dying during whelping.
I have been around long enough to unequivocally say I will NEVER EVER agree with any breeder, breeding a 3lb. 3.5lb. dog...Absolutely zero good reason to do so, and a million reasons NOT to.