Has the Doctor ever been a woman in Doctor Who?

Yes, the Doctor could simply be messing with Bill, but if he is being serious it suggests that either he remembers that he's been female at some point in his life but can't recall exactly when or that he can't actually remember whether he has been or not (after all, your memory's bound to suffer after a couple of thousand years). Either way, it leaves open the possibility that the Doctor was indeed once a woman (whether he knows it or not).

But, of course, there's a snag – we already know all of the Doctor’s regenerations, and they’re all men. In both The Name of the Doctor and The Day of the Doctor all 13 versions have appeared onscreen, with no sign of a woman among them.

In order, the Doctors are: William Hartnell, Patrick Troughton, Jon Pertwee, Tom Baker, Peter Davison, Colin Baker, Sylvester McCoy, Paul McGann, John Hurt, Christopher Eccleston, David Tennant, Matt Smith and Peter Capaldi. Nope, no women there – and within the basic rules of regeneration, there shouldn’t have been any more.

According to Whoniverse mythology Time Lords can only regenerate 12 times (which the Doctor had actually done by the time of Matt Smith’s incarnation, thanks to a double regeneration for Ten). So there can be no secret female Doctors in there; unless all the rules of regeneration were broken, of course. Now, where have we heard that before…

Oh yes, that’s right – 2013’s Time of the Doctor, when the Time Lords gave the Doctor a whole new cycle of regenerations to save his life and change him into Peter Capaldi’s version of the character. And he’s not the only Time Lord to circumvent the 12 regenerations rule – back in the classic series, The Master (now Missy) managed to steal the regenerations of another Time Lord, and after his death was later resurrected for the Time War with yet another cycle granted him.

So who’s to say a new round of regenerations couldn’t have been bestowed upon the Doctor once before? Just prior to him regenerating into William Hartnell’s Doctor, in fact, before he ran away from Gallifrey – back to a time he’d rather forget – perhaps explaining why it never comes up in the series?

Just take a look at this clip from classic episode The Brain of Morbius in which the Fourth Doctor takes part in a psychic duel with the eponymous villain. The apparatus they are using begins cycling backwards through the Doctor's former faces as Morbius asks "How far Doctor? How long have you lived? Back, back to your beginning..."

But when we get to William Hartnell's supposed First Doctor, the process continues and we see eight more faces, apparently in period dress, before the intensity of the contest causes the machine to explode.

This points squarely at a previous cycle of regenerations for the Doctor and although some people have argued that the faces could belong to Morbius, episode producer Philip Hinchcliffe says the intention was absolutely to imply that they are former Doctors.

"I just reasoned that it was entirely possible that William Hartnell may not have been the first Doctor Who," says Hinchcliffe. "So yes, as far as [writer] Bob [Holmes] and I were concerned, the other faces were meant to be past Doctors... it is true to say that I attempted to imply that William Hartnell was not the first Doctor."

Of course, if the Doctor has lived through another cycle of 13 faces and we've only seen eight of them, that means one or more of the previous five may indeed have been female.

Although, given that at least 21 of the Doctor's 26 faces so far have been men, at this point you have to wonder whether he has something against ending up in a female body...