Swyer syndrome is also known as XY gonadal dysgenesis. The testes fail to develop but there are no symptoms until puberty fails to develop in the early teens. These children appear to be girls as the penis does not develop. Only when puberty fails and genetic tests are done are they found to be genetically males (XY) and not female (XX).
The remnant testes are very likely to become cancerous so are usually surgically removed.
Once Swyer Syndrome is diagnosed, oestrogen is given to complete the feminisation of the patient, allowing breast and pubic hair development and widening of the pelvis. The oestrogen must be continued lifelong as a patch or pill. Progesterone may also be given to balance the oestrogen. In some patients there may be a vagina and uterus present, but pregnancy is not possible.