spay as she is too small and for sure would have a c section i am sure and those are not cheap and very hard on your baby - also do before first heat as pyometra has killed two yorkies on my other groups because the girls were not spay - it happens fast and if not caught quickly they can die -- see link below about it
Pyometra & Uterus Infections in Dogs Spaying (Ovariohysterectomy) Female Dogs