Can A Dog Get Pregnant After Only Breeding Once With A Male?

Yes,A Dog Gets Pregnant After Only Breeding Once With A Male

Yes. We count the days from the first tie. ;-) Breeders only allow the tie or inseminate 1 to 3 times or so in general (not in a day though if more than once). No need and could be detrimental if more often. And if another male somehow gets her then you can have multiple sires and DNA matching to show who belongs to who. If she didn't take there is likely an underlying issue with one or both dogs (health, STD, ect). If you had an accident you can speak to your vet about the mismate shot and it's risks (pyometra and spay) if soon enough, a spay and termination or if you want to continue with the pregnancy you can call and set up your first prenatal care appointment which is confirmation by ultrasound or a quick blood test.
www.askabreeder.com  
http://drnelsonsveterinaryblog.com/2009/01/01/canine-labor--when-to-call-the-vet.aspx  (most go by 67 or 68 and not 72)
www.learntobreed.com  
www.dogbreedinfo.com  
www.bullmastiffsonline.com
www.marvistavet.com
www.dogfoodanalysis.com