What is the ruling on swearing an oath by divorce upon one's wife to change the doctor monitoring her pregnancy, and what is its expiation?

The phrase mentioned in the question is an allusion to divorce, not an explicit one. If the intention behind it was a threat or similar, then no divorce occurs, no expiation is required, and there is no blame for breaking the oath for a good reason. However, if the intention was to suspend the divorce on the wife giving birth at another doctor's, then divorce occurs. If the intention of the oath was to prevent her from going to the doctor, then divorce occurs as soon as she goes, and the waiting period (iddah) ends with childbirth, and she cannot return except with a new marriage contract. But if the intention was to suspend the divorce on the occurrence of childbirth at the doctor's, and not merely going there, then divorce does not occur until childbirth is complete, and the husband can take her back during the iddah without a new contract. Some scholars, like Ibn Taymiyyah, hold the view that if the husband retracts the suspension of divorce after the wife has gone to the nearby doctor, then divorce does not occur if she goes to another doctor and gives birth there.