Christine Jorgensen, formerly George Jorgensen, was an ex-GI and one of the first Americans to undergo surgical gender reassignment.

Roger Jackson/Central Press/Getty Images

As you were growing up, how did you know you were a girl or a boy? Was it because you had a vagina or a penis? Or was it something other than than what your body looked like? Some people feel that their minds and bodies don't match up. This feeling is commonly known as gender identity disorder or gender dysphoria. The medical community used to use the word "transsexuals" to describe this community. But today, "transgender" is used as a general, non-medical term to describe anyone whose gender identity is different from their physical sex at birth.

Transgender people often wish to live as a different gender than the one they were assigned when they were born. They may may transform their bodies through gender reassignment surgery, also known as gender confirmation surgery -- a collection of procedures also commonly known as a "sex change." The word "transition" describes a person's shift to living as the gender they perceive themselves to be, rather than the sex they were assigned at birth. Surgery is sometimes, but not always, part of a person's transition.

Gender identity struggles usually begin in early childhood but have been identified in people of all ages. A biologically born man who identifies as a woman is known as transwoman, or transgender woman. A biologically born woman who identifies as a man is known as transman, or transgender man. There are also people whose identity lies somewhere along a spectrum of gender who may refer to themselves as neither male nor female.

It's estimated that one in 11,900 males and one in 30,400 females are transgender adults [source: WPATH Standards of Care]. Lynn Conway, a professor emerita at the University of Michigan, estimates that one in 2,500 United States citizens has undergone male-to-female gender reassignment surgery [source: Advocate].

One of the most publicized Americans to undergo such surgery was Christine Jorgensen, who traveled to Denmark in 1952 to undergo an early gender reassignment surgery. Genital reassignment surgery wasn't performed in the U.S. until 1966. Jorgensen later worked with Dr. Harry Benjamin, the physician who coined the term "transsexual." Benjamin was one of the pioneering doctors to research and work with gender identity disorders, using the research of Magnus Hirschfeld, of the Institute for Sexual Science, and Alfred Kinsey, of the Kinsey Institute, as his springboard.

­In 1966, Benjamin published "The Transsexual Phenomenon" and went on to establish the Harry Benjamin International Gender Dysphoria Association, Inc. (HBIGDA). Today HBIGDA is known as the World Professional Association for Transgender Health (WPATH), and is an international organization devoted to furthering the understanding and treatment of gender identity disorders. WPATH established and still publishes the Standards of Care (SOC) for the treatment of gender identity disorders. It also publishes ethical guidelines for professionals caring for transsexual patients.

So what are the requirements for gender reassignment surgery? Are patients fertile and able to have sex? Go to the next page to find out what happens before a person physically changes genders.