My mini dachshund is on her 12th day of her cycle. We have tried to keep her and another family male dachshund separated but twice now I think they have mated. There was no "lock" but I know (according to a veterinarian) there does not have to be for her to get pregnant. This was not planned and I did not want her to have any puppies at this time. Are there any signs at this early stage that could tell me if indeed she was impregnated?

There really isn't much that could tell you at this early stage whether she's pregnant.

I would take her to the vet. If she is indeed pregnant you can get a spay/abort done.

SOME b!tches heats will suddenly end if impregnated but it's not a surefire way to tell.

There are a few options. How long ago did this happen? If you can get to the vet within 48 hours of the mating you can get a misalignment injection... it's basically "Plan B" for dogs.

If it has been more than 48 hours you can get a spay/abort done as Alty said. At this early stage there is no more risk than a regular spay.

None this early. Sorry