Yorkies referred to as allegedly teacups by uninformed and not very reputable breeders are actually undersized representatives of the breed. No true Yorkie fancier would ever consider purposely breeding them or breeding for them nor using them for stud. An undersized dog of any breed and particularly a toy breed such as the Yorkie, will most of the time present with a myriad of health problems causing astronomical Vet bills. Some live a long life many don't.
Here is more info from the Yorkshire Terrier parent club Yorkshire Terrier Club of America.
http://www.ytca.org/faq.html#A
I agree with many of the posts here, you better be well versed in what you are doing before you ever begin to think of breeding your girl. The risks to her life are always there when a bitch is bred and whelping a litter.
You have to be prepared in the event of losing your bitch, losing bitch and puppies, problems in rearing meaning you are up tending to puppies around the clock every 90 minutes for about three weeks.
There is a large responsibility in breeding responsibly.