if you are not a fully experienced breeder breeding under direct supervision from another breeder who has been doing this for years then you really have no business breeding your dog. breeding is a very difficult endeavor that takes a ton of knowledge that it takes years to learn. unless your dog is of show quality and comes from a long line of yorkies with genetic testing and good sound bodies and health then she needs to be spayed and not bred. leave the breeding to those that have been doing it all their lives and if you want to learn about it in the future find a breeder close to you on
www.ytca.org and have them mentor you in the proper breeding techniques and practices before you ever try it on your own.
please spare the world more yorkies being bred for "fun" or "Money" and don't breed this poor girl, it's her life you are taking into your hands and she doesn't deserve to loose her life over your wishes to have pups off her.