Could my dog be pregnant

We went to the vet when my dog would have been on day 40 of her pregnancy and the xray didn't show any puppies.  The vet said she felt pregnant, nipples really enlarged, abdomen getting bigger, vulva enlarged.  Her behavior has changed a lot.  She is on Heartguard and frontline regularly and not missed any does.  I'm sure she is pregnant.  If it is a false pregnancy would her abdomen be getting significantly bigger like it is? Is 40 days to early to see puppies with an xray and what should I do now?

[Holliee]

You should be able to see puppies mine just had some and it was about day 48 or 50 and we could see her's maybe go to another place get scan/exray

[ginger899]

Why didn't your vet do an ultrasound? Exactly the same as with a human fetus?

I might be wrong here, but I have never known a false pregnancy to go on so long. It usually lasts a week or two, and although there are behaviour changes, and some swelling of teats and sometimes even milk or a clear fluid from the nipples, there is not usually marked abdominal swelling.
40 days is 2/3 of the way through the pregnancy. I would think by now pup fetuses would be noticed on ultrasound.

[have 2 kids]

My dog had a false pregnancy.   She developed milk and enlarged teats close to what would have been day 55-60.    Her abdomen did get a little bigger but not alot.  She even began nesting.  

Our vet has always told us that x-ray won't show puppies until about 2 wks before the puppies are due.  This was 15 yrs ago so things could be much more advanced now.
I would think they would definately show up on an ultrasound by now.