My Labs Have Been Stuck Together Three Times What Are The Chances She Is Pregnant?

Regardless of whether any standard protocol was followed she is unless there is a problem or you missed a test on one or both of the dog's (STD, thyroid, hormone ect.). Now the trick is to keep her away from all other dog's (including him) until there is no possible chance it will happen again (often longer than many think).  Ultrasound can be done about 21 days in (though waiting a little longer, even just to 25 or 26 is often beneficial) and if your vet doesn't have one you can run a quick blood test to confirm earlier and then do your x-ray's day 45 or later. Good Luck.

http://drnelsonsveterinaryblog.com/2009/01/01/canine-labor--when-to-call-the-vet.aspx
www.dogchannel.com
www.justanswer.com
www.thelabradorclub.com
www.learntobreed.com
www.dogbreedinfo.com
www.marvistavet.com
www.dogchannel.com
www.askabreeder.com
www.dogfoodanalysis.com