Determining the date of birth?

Question
We inherited and hopefully saved (long story) a Bullmastiff that's about 2 yrs old and pregnant (as long as it's not a false pregnancy) but the date of breeding is questionable. Due to the country I'm in - dates are relative here - so 5 weeks ago can also mean 6 or 7 weeks ago. Another words, 5 weeks is more or less the time frame "he thinks." We've only had her 2 days and by our best estimation she's probably 6-7 weeks pregnant.

Her nipples are in and are full but no milk yet. (Are there other signs to show her potential gestational age?)

Today 2 things happened that have me concerned that my dates may be off. First her vulva seems much more pronounced today and 2 she's panting very hard ... especially tonight with no activity. She ate good at dinner. No vomit or anything else to indicate labor ... but she may be in first stage of labor or I may be totally off. This is both her and my first litter.

In the morning (if we don't have this litter tonight and pray we don't because we aren't prepared tonight at all - no whelping box and she's so knew to our home she won't come inside and it's pretty cold outside) we're going to the vet to have her xrayed so we at least know how many to expect (and rule out false pregnancy).

All of this background to ask ... Is there any way for the vet to better determine the potential gestational age?

Any and all help on all whelping matters is appreciated.
-From a nervous and expectant Dad (owner), Tim

Answer
Hi Tim

If you are taking her to the vet for x-rays, you should know that calcification in pups doesn't occur until 50 days.  What that means is x-rays won't pick up much before that time, so the vet will be doing guess work.  

Some things to look for is a leaking, which indicate her mucus plug has discharged.   A lack of appetite and nesting are all signs that she is close.  

Good luck