The causes of gender dysphoria are not fully understood.
Gender dysphoria was traditionally thought to be a psychiatric condition, which meant that its causes were believed to originate in the mind. This does not mean that gender dysphoria is a mental illness.
However, recent studies have suggested that gender dysphoria may have biological causes associated with the development of gender identity before birth.
More research is needed before the causes of gender dysphoria can be fully understood.
Typical gender development
Much of the development that determines your gender identity - that is, the gender that you feel yourself to be - happens in the womb (uterus).
Your biological sex is determined by chromosomes. Chromosomes are the parts of a cell that contain genes (units of genetic material that determine your characteristics). You have two sex chromosomes: one from your mother and one from your father.
During early pregnancy, all unborn babies are female because only the female sex chromosome (the X chromosome), that is inherited from the mother, is active. At the eighth week of gestation, the sex chromosome that is inherited from the father becomes active, this can be either an X chromosome (female) or a Y chromosome (male).
If the sex chromosome that is inherited from the father is X, the unborn baby (foetus) will continue to develop as female with a surge of female hormones. The female hormones work in harmony on the brain, gonads (sex organs), genitals and reproductive organs, so that the sex and gender are both female.
If the sex chromosome that is inherited from the father is Y, the foetus will go on to develop as biologically male. The Y chromosome causes a surge of testosterone and other male hormones, which initiates the development of male characteristics, such as testes. The testosterone and other hormones work in harmony on the brain, gonads (sex organs) and genitals, so that the sex and gender are both male.
Therefore, in most cases, a female baby has XX chromosomes and a male baby has XY chromosomes.
Changes to gender development
Gender development is complex and there are many possible variations that can cause feelings of a mismatch between a person's biological sex and their gender identity.
In rare cases, the hormones that trigger the development of sex and gender may not work properly on the brain, gonads and genitals, causing variations between them. For example, the biological sex (as determined physically by the gonads and genitals) could be male, while the gender identity (as determined by the brain) could be female.
This could be caused by additional hormones in the mother's system, or by the foetus's insensitivity to the hormones, known as androgen insensitivity syndrome (AIS). In this way, gender dysphoria may be caused by hormones not working properly within the womb.
See androgen insensitivity syndrome for more information about this condition.
Other rare conditions
Other rare conditions, such as congenital adrenal hyperplasia (CAH), and intersex conditions (also known as hermaphroditism) may also result in gender dysphoria.
In CAH, the adrenal glands (two small, triangular-shaped glands located above the kidneys) in a female foetus cause a high level of male hormones to be produced. This enlarges the female genitals. In some cases, they may be so enlarged that the baby is thought to be biologically male when she is born.
Intersex conditions cause babies to be born with the genitalia of both sexes (or ambiguous genitalia). In such cases, it used to be recommended that the child's parents should choose which gender to bring up their child as. However, it is now thought to be better to wait until the child can choose their own gender identity before any surgery is carried out to confirm it.
Two small, triangular-shaped glands that sit on top of the kidneys, high up inside the back of the abdominal wall. They produce adrenaline, steroid hormones and the male and female sex hormones, testosterone and oestrogen.
Chromosomes are the parts of a body cell that carry genes. A human cell usually has 23 pairs of chromosomes.
A foetus is an unborn baby, from the eighth week of pregnancy until birth.
In this article, gender refers to the feeling of being either male or female.
A condition that describes the feeling of mismatch between your biological sex and your gender identity.
Gender identity is your personal sense of knowing which gender you belong to, or the way that you see yourself.
A gene is a unit of genetic material that determines your body's characteristics.
Hormones are powerful chemicals that are produced by the body and have a wide range of effects.
In this article, sex refers to male or female, the biological sex that you were born with.
A transsexual is someone with an extreme and long-term case of gender dysphoria, who seeks to alter their biological sex to match their gender identity.
The uterus (also known as the womb) is a hollow, pear-shaped organ in a woman where a baby grows during pregnancy.