If the dog is pregnant and it is undesired, there are many ways to perform a safe pregnancy termination under the guidance of a medical professional. Dogs can also experience spontaneous abortions or miscarriages for a variety of medical reasons.
If the dog has experienced a spontaneous abortion, an abnormal vaginal bleeding would be seen and in some cases an expelled fetus may be found. The most common cause of a spontaneous abortion is fetal death due to a hormonal imbalance.
In the case of a planned abortion, bleeding happens following the procedure. The dog needs to be closely monitored so that any side effects or health related issues can be responded to quickly.