I am no breeder, but you should know that breeding Yorkies is very dangerous, as you could lose your female or pups.
If in fact she has conceived, her due date is fast approaching and you have very little time to prepare. You can expect pups anywhere from 21st to the 25th, if the date you have given is the first and only time she was bred. If she was bred before that date and gone unnoticed, she will deliver before then.
Do you have any experience with whelping yorkies? Do you have a mentor or someone you can call that is well versed with small breeds?
You should contact your Vet ASAP and get her in to be seen. Getting a Xray is very important as others have stated. It will be very informative in letting you know the size and number of pups and can also help determine if she able to have a free whelp.
Here is a website that is very informative.
Whelping Puppies, Breeding Dogs
There are many great breeders on YT that will offer assistance. If you will do a search of older posts you will find alot of great information on the whelping process, before and after care, signs of complications, and what to do should they arise.
Good luck with your girl.

YorkieProud