Swyer syndrome (XY gonadal dysgenesis)

Swyer syndrome is a Disorder (or Difference) of Sexual Development (DSD) condition affecting the sexual development of a chromosomally male foetus, resulting in typically female genitals and reproductive organs, but non-functional ovaries. Sexual development does not fit in with the person’s chromosomes. It occurs in about one in every 80,000 live births.


Genetic female – 46, XX – female appearance and organs
Genetic male – 46, XY – male appearance and organs

Swyer syndrome

Genetic female – 46, XX – female appearance and organs
Genetic male – 46, XY – female appearance and all female organs except ovaries

What Swyer syndrome looks like in real life

People with Swyer syndrome have typical-looking female genitals, so the labia and clitoris all look and behave normally. The uterus and fallopian tubes form normally, but the ovaries are non-functional. Instead of ovaries or testes, what’s known as streak gonads exist in their place. Physically, women with Swyer syndrome can be taller than average, with a smaller uterus, and a slightly enlarged clitoris.

Unfortunately remaining gonadal tissue easily becomes cancerous, so must be detected and removed as soon as possible. Cancers do grow in children before anyone realises what the problem is. These cancers are benign, and known as gonadoblastoma.

Swyer syndrome is usually an isolated sex development issue, but from time to time it can affect the nerves (neuropathy) or arrive as part of another disorder called campomelic dysplasia, causing skeletal abnormalities.

Gender identity

People with Swyer syndrome are usually raised as girls and have a female gender identity. Hormone replacement therapy is usually started during teenage years to bring on the menstrual period and breast and uterine growth. Hormone replacement therapy is required because the ovaries would normally produce oestrogen, but if they do not exist, they can’t. Artificial means are the only way to produce a normal female body into adulthood. The only other option is no puberty, complete infertility, and essentially switching straight from childhood into menopause.


Bone density relies on our sex hormones, so hormone replacement therapy is required to maintain bone density into old age.


Women with Swyer syndrome don’t have their own eggs, but because their uterus is fully functional, a donated egg can be used to get pregnant.

Why does this happen?

Some hormonal medications taken by the mother during pregnancy (particularly early pregnancy) have been associated with this condition, but apart from the genetic cases – of which do not explain all cases – we don’t know why this happens. It does not appear to run in families, though the gene mutations can be inherited.


In some people, Swyer syndrome is caused by a mutation in the SRY gene – about 15 per cent. The SRY gene is located on the Y chromosome, and gives orders regarding sex determination of a foetus. This is undertaken by sex-determining region Y protein. Sex determination is different from genetic ‘gender’, which is determined by the sperm – you are supposed to be either XX (female) or XY (male). This obviously doesn’t always work out that well, due to a variety of genetic mutations that are out of our control.

If a foetus does not produce sex-determining region Y protein, it will not develop testes, but instead a uterus and fallopian tubes. If a foetus does produce this protein, normal male characteristics will develop and nobody is any the wiser.

In about 18 per cent of cases, a different mutation is responsible (the MAP3K1 gene). This gene gives directions for the creation of a protein that assists in regulation of signalling pathways that control some body processes – including the determination of our sexual characteristics in the foetus. Signalling is decreased that would normally lead to male sexual characteristics, and because female characteristics are the default, without signals to the contrary, a mostly-female is created.

Then there are mutations in the DHH and NR5A1 genes, but these are less common. Both affect the process of sexual differentiation and prevent affected foetuses from developing testes, and resulting in the growth of the default uterus and fallopian tubes.


Most people with Swyer syndrome don’t have any symptoms that would signal an issue until teenage years, when puberty doesn’t appear, including menstrual periods failing to arrive (primary amenorrhoea). This is because there are no ovaries, and therefore no oestrogen to trigger the process. Breasts don’t develop, pubic hair is absent, and other puberty signs like enlargement of labia don’t occur, and the girl remains girl-like in appearance.

Once hormone therapy is started, breasts, pubic hair and all other puberty signs appear, including the growth of the uterus and development of internal and external sexual characteristics.

Swyer syndrome may also be called:

  • 6,XY CGD
  • 46,XY complete gonadal dysgenesis
  • 46,XY sex reversal
  • Gonadal dysgenesis, 46,XY
  • Gonadal dysgenesis, XY female type
  • Pure gonadal dysgenesis 46,XY
  • XY pure gonadal dysgenesis