Your female can get pregnant during her first heat (at 6-12 months) but it is certainly not advisable. Breeding at any age carries a lot of risk and should only be done by experience breeders. Would it be worth taking a chance on losing your precious girl? There is much to consider and a lot of responsibility.
Here is some information from the YorkieTalk Library on breeding.
http://www.yorkietalk.com/forums/bre...t-breeder.html http://www.yorkietalk.com/forums/bre...-answered.html