Liquidgirl - I fretted about doing this too, (x-ray/scan) and decided after advice, not to.
Tammy is day 39 today, and I've no idea how many puppies she has in her - I will just do whatever I need to at the time, rather than risk her to stress and infection at the vets now.
How heavy is your Yorkie usually ?
p.s. I saw your pregnant pictures of Lilly, and she is gorgeous - do me a favour and put them on my PREGNANCY PICTURES PLEASE thread....so we can see how they are all doing together.
Thanks ! 
Debra x x x x
pics 1+2: Tammy today - 3rd April
pics 3+4: My three babies, Tara, Tammy and Casper(Dad to be)