There is a "code of ethics" that breeders use when they choose their bitches to breed. Of this code of ethics, there are many different tests that a prospective dam must pass before she should be bred. These might include:
- Required vaccines
- Hip and eye validation
- Blood work to determine organ function
- AND TESTING FOR DISEASE
If any of the tests for disease come back positive, the bitch should not be bred at that time.
There certainly is no hard and fast rule but it's just bad breeding practice to breed a dog that is on antibiotics and has tested positive for lyme disease.
With a few exceptions, one should avoid using all medications in pregnant and lactating (nursing) dogs. Doxycycline has been used effectively during pregnancy if it is needed and cannot be avoided but you should not knowingly breed when a dog is on a long term medication.
You can probably tell how strongly I feel about this subject. I do hope that you understand that I am not only trying to educate you about good breeding practices but am also trying to help your future pups.
As a side note, there are so many dogs out there that need homes that I don't personally feel like anyone should be breeding substandard pups just because they can.
If you have more questions or you need more specific answers, please feel free to ask. There are a lot of points here that I have hit on but I may have missed something that you still want to know.