This is a question most of us have asked ourselves. Dick Swaab, a Dutch Neurosurgeon writes in his book “We are our brains: from the womb to Alzheimer’s”, that:
“MtF occurs in 1 in 10 000 individuals and FtM in 1 in 30 000. Gender problems tend to become apparent from an early age. ….. All the data indicates that gender problems arise in the womb. Tiny variations in genes associated with the effect of hormones on brain development have been found to increase the likelihood of transsexuality. It can also be increased by abnormal fetal hormone levels or by medication taken during pregnancy that inhibits the breakdown of sex hormones. The differentiation of our sex organs takes place in the first months of pregnancy, while the sexual differentiation of the brain occurs in the second half of pregnancy. Since these two processes take place at different times, the theory is that in the case of transsexuality, they have been influenced independently of one another.” (Pg. 68).