[quote=Kaga]
No, it's been implied that regeneration can change a Time Lord's sex. I believe it was in the episode "The Doctor's Wife" when the Doctor was talking about a Time Lord he knew once and reminiscing about their adventures - he referred to them as "he" most of the time but at one point he said "or she, in one regeneration" or something similar. I don't remember the specifics but I'm certain it came up. Additionally, immediately after the Doctor regenerated into Eleven, he momentarily thought he might've become female after realizing how long his hair was. I don't know if it's a good idea for the Doctor himself to regenerate into a woman at some point, but it is canonically possible. If they do go that route, they'd probably add to the lore saying it's rare for a regeneration to do that, since it's never come up before except in those two instances (that I'm aware of). Now that I think about it, I wouldn't mind a female Doctor coming in similarly to how the War Doctor did - not taking up entire seasons but just one movie or special. That's probably the best dose at which to deliver something so different.
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Oh...

In that case, bring on the female doctor! :P