Particularly for a pregnant xray it's very important that the dog be absolutely still otherwise the xray will be blurred and they can't count puppies very effectively. Some of the positions necessary for an xray can be very uncomfortable for the dog. Because of both of those reasons some dogs need to be sedated some.

If you have concerns about your dog being sedated while pregnant, talk to your vet. Part of their job is to help you understand what they are doing with your pet and why. I'm sure your vet would not mind one bit to discuss any concerns with him/her so that you have full confidence in them and know what your pet will experience.

BTW here's an xray I found online of a pregnant dog. Thought you might want to see what it would look like.

pregnant dog xray