The rare genetic disorder is caused by a missing enzyme which prevents the production of a type of male sex hormone - dihydro-testosterone - in the womb.
All babies in the womb, whether male or female, have internal glands known as gonads and a small bump between their legs called a tubercle.
At around eight weeks old, male babies who carry the Y chromosome start to produce dihydro-testosterone in large amounts - this turns the tubercle into a penis. For females, it becomes a clitoris.
But some male babies are missing the enzyme 5-α-reductase which triggers the hormone surge. This means they appear to be born female with no testes and what looks very much like a vagina.
Then when puberty hits, a large surge of testosterone is triggers the male reproductive organs to grow. This causes their voices to deepen and the development of a penis.
Essentially, the development that should have taken place in the womb with these children happens around 12 years later.
