[luckylady]

Yes, the stakes are higher for possible problems with smaller pregnant Yorkies, but many whelp with no problems. I would guess that if your vet thought it was necessary to schedule a c section after seeing the X-ray, it would have been suggested to you at that time. Have your vet on call when she goes into labor. And if possible have someone with you with some experience. As for signs of labor..........most will refuse their meal(s) the day of or before, their temp will drop in the 98 degree range or you will notice that her belly feels a lot cooler than it has been during pregnancy. As she gets closer to delivering, she will dig in blankets, her bed, or the floor, and may need to go potty more often. Noticible contractions will come next.......you will know what they are when you see them, they're pretty obvious. Stay with her at all times, stay calm, help her the best you can, and most importantly, if your gut/heart tells you something is not right, get her to your vet or emergency vet ASAP.
I had a female with a singleton puppy her first litter. After seeing the X-ray my vet came in and told me, " you have one very large puppy". Scared me to death! My female weighed in the 6 pound range, so no where near as little as your girl, however her puppy weighed 6 3/4 ounces at birth and that is a big puppy for any yorkie. It took her a bit of work and a little help from me getting the head out, but she did have him naturally.
A good way to estimate when she is due would be to count 63 days from the time you left her with your friend starting with the day you dropped her off and including the last day of her visit. It's been my experience with X-rays that pups were born usually a week and a half later. Good luck to you and your girl, I truly hope all goes well. Please keep us posted and let us know.