I think the Doctor got extra regenerations when he saved Gallifrey. Other Time Lords have been mentioned as switching genders.I have a question. Timelords according to canon are suppose to only live 12 lives or is that wrong? She is suppose to be breaking that cycle for a reason I can't recall. Basically doesn't this mean that this would not be a normal regeneration but a kind of physical change/regeneration unlike all the rest?. Maybe the Doctor was never suppose to be a women for whatever genetic reasons but once he/she moved past their 12 lives it sort of changed things where he/she can now be both genders. A kind of added feature that he use to not be able to do but now can.
Jason