Well, I haven't bred yorkies yet, but with other dogs I have noticed that their vulva will stay swollen and not go down to pre-heat size. I don't know if thats all dogs, but it was with at least one of mine. Also, just a couple of weeks in I have noticed swelling of the teats a little with a general appearance of not feeling great. I have only ever had one dog in my life actually experience morning sickness, and I want to say that was about 3-4 weeks along. Sometimes physical signs are not exceptionally evident though until just a couple of weeks before delivery for some dogs.
