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Hi, I am Dr. Peter and would like to help.
1- When did the possible breeding happen?
2- Are you considering breeding her or spaying her?
Being pregnant on the first heat is not ideal. But, most dogs will carry the pregnancy to term without any problems; no need to be concern. Another concern would be if the father was much bigger the pups may be to large for her pelvic canal and she may need a c-section. However, at 3 weeks she is still in time for you to schedule her and get spayed; most vets will spay her at 3 weeks of a possible pregnancy, not a problem.
If you have additional questions I will be glad to answer them.