That’s distinctly different from ‘original gender’ for two reasons. First, sex and gender are not the same thing. Basically, gender is psychological (gender identity) and sex is anatomical (reproductive organs). For most people that distinction isn’t important because their gender identity matches their reproductive organs. But for transgender people the distinction between gender and sex is extremely important because their gender identity is different than their reproductive organs. The very definition of transgender requires you to distinguish between sex and gender. And the reason why this is especially important to your story is that as far as we know, a person’s gender identity is programmed in utero and remains fixed for their entire life [ see reference: Bao and Swaab, 2011]. So, for example, in the case of a transgender woman with male reproductive organs and a female gender identity, their ‘original gender’ would be female, but their ‘original sex’ would be male.