Can a doctor perform an operation on his patient without his consent?
Yes, if it is very urgent and if the patient is in no condition to give his consent. In cases of emergency or unconsciousness (of the patient), no consent is required and the doctor can do whatever is necessary to save the patient's life. However it is to be noted whatever the doctor does in such situations must be in the best interests of the patient.
In life-threatening situations, doctors are not required to consult with parents or guardians of minor patients. The normal rule is that a doctor must seek the permission or consent of the parent or guardian of a minor patient before performing any risky operation.
Note: Causing miscarriage is an offence even if it is done with the consent of the woman or her guardian. But if it is done to save the life of the woman, it will not be an offence (S. 91 of IPC).