
I have a toy breed, average litter size is 1-3 and mine often don't look pregnant until the last minute! I've had a pup stay under the ribcage until the day before whelping, and don't usually feel movement until the last week.
Yet last year I mated a maiden bitch, and she worried me to death. I didn't need a scan or confirmation, I could see she was pregnant from very early on. By 5 weeks her belly was nearly dragging the floor. She is a small bitch, but had my vet check out her pelvis on several visits prior to making the decision to breed. She looked positively gross.
I have never had a bitch whelp so easily, I've seen stronger hiccups! She had FIVE pups, and I had a seizure!
No problems whelping at all. But, with any doubts, I agree, ask the vet to give her the once over. I would rather the vet said I was wasting his time, than miss something that I shouldn't have.