I recently read a wonderfully informative answer to the very general but valid question: What causes a person to be transgender? found on the Q & A forum-style website called Quora. I’m not sure there is a rock-hard, definitive answer to this question; however, Ms. Williams—an MTF trans woman herself—eloquently and humbly touches upon the prevailing theory that posits transgender begins in the womb and results from hormonal influxes occurring at perhaps inopportune moments. She does right to include several links to additional information—somewhat of a “Notes” section, if you will. Please find the link to this below.
Important Note: Ms. Williams’ response to this question is the second answer from the top. Be sure to scroll down just a bit, and you’ll find her response.