Can my two breeds mate and survive

Question
Hello Ma'am,
I have a 2y/0 male St. Bernard 101lbs. I also have a 9 month old female Boston terrier/pug mix 20lbs. She is in heat and our male is going crazy. My question is can she get pregnant and survive the birth of puppies being the male is so big? We let them out side together and thought she was younger than that when we bought her. I noted some blood and I am almost positive she menstrated. I am nervous for her and am hoping that if they did mate she will survive.

Answer
Hi Kristine:

While it's scientifically "possible" for a mating between two such breeds to occur I'd really consider your particular situation unlikely. Aside from the obvious size difference & the problematic potential for accomplishing mating, plus you neither witnessed him mounting or a tie, and if your female is still having a bloody vaginal discharge, it's likely she's not yet in estrus, which is the fertile portion of the heat cycle. This usually begins around day 9 or 10 from the initial onset of the cycle & the vaginal flow will become lighter in flow & color, becoming more of a light pink to straw color. The estrus portion of the cycle lasts anywhere from 4 to 12 days as all dogs vary. It's perfectly for your female to have had her first heat cycle at the age of 9 months - the average is between 6 months to a year. Worst case scenario, however -- if she is/was in estrus & they did mate I'd strongly urge you to discuss having the pregnancy terminated with your vet. In your case I think the simplest & least expensive way to go about it would be to have her spayed ASAP -- some vets prefer not to spay while the dog is still in heat & if that's the case, keep them separated for another 2 to 3 weeks to be on the safe side. Chances are she's not pregnant but spaying will prevent any future accidental breedings as well as increase your girl's chances for a longer, happier life. If she were pregnant & you opted to proceed with the pregnancy you'd almost certainly be looking at the very least a delivery via C-section & possibly some additional health risks for both Mom & offspring.