I always ultrasound my girls at 4 weeks from the last breeding. My Vet has missed twice and both times they were small litters. I find my girls tend to carry their puppies up high.
At 4 weeks we can see heart beats and my Vet never shaves their belly. I don't routinely x-ray for a head count as I have never been given the correct number of puppies...seems like they always miss one or two, especially with a big litter.
Lou Ann