Thirteenth Doctor (The Curse of Fatal Death)

In a universe other than the one that the Renegade Time Lord known as the Doctor called home, the Thirteenth Doctor was the result of the regeneration of the Twelfth Doctor.

Biography
Her previous incarnation regenerated after being hit by zectronic energy.

With the Doctor now regenerated into a woman, she was abandoned by her companion and lover Emma and "his" previous plans to marry her were cancelled. However, the Doctor and the Master found each other much more attractive than before and walked off together as a couple. (TV: The Curse of Fatal Death)

Behind the scenes

 * The inclusion of a woman as the Thirteenth Doctor addressed long standing fan speculation that the Doctor could at some point change sexes while regenerating. The theory has been supported by Tom Baker. The first direct confirmation that a Time Lord can change sex later came in TV: The Doctor's Wife, where the Doctor mentioned that he had known the Corsair in various male and female incarnations.
 * Fans had for years thought Joanna Lumley a candidate for the role of the Doctor, should they ever cast a woman in the role. She had earlier played Sapphire, a non-human time detective in the cult fantasy series Sapphire & Steel. The script gave her name as "The (Female) Thirteenth Doctor".
 * Another female incarnation of the Doctor would later appear in NOTDWU: Exile, played by Arabella Weir.