By Caroline Parkinson
Health reporter, BBC News
The birth of a baby girl to a Thomas Beatie in the US has, not surprisingly, drawn international attention.
Mr Beatie stopped hormone treatment in order to be able to get pregnant
People have wondered how Mr Beatie, 34, could possibly have a child.
The answer is that although he is now legally male, Mr Beatie was born a woman - Tracy Lagondino.
While he has had his breasts removed and has taken male hormones to give him facial hair, he has kept his female reproductive organs and can therefore carry a baby.
His daughter was conceived via donor insemination after he had stopped taking his hormone treatment.
Dr James Barrett is a psychiatrist based at the National Gender Identity Clinic in London, which sees over 700 patients a year seeking gender reassignment.
He said it was unusual for the female hormonal system of someone who had taken male hormones to restart, as Mr Beatie's did, in order to allow periods and pregnancy to take place.
Dr Barrett also explained the process a patient seeking gender reassignment in the UK would go through.
STAGE 1 - DIAGNOSIS
The first stage is for someone to be diagnosed with transsexualism.
While a patient can come to a clinic wanting gender reassignment, it may be decided they have another condition, such as a rare endocrine disorder, or that they are gay but have difficulty coming to terms with it.
The diagnosis is usually made by a psychiatrist.
Counselling is offered to patients before, during and after any treatment.
STAGE 2 - REAL LIFE
A patient then has to live for a significant period of time in their chosen gender.
They need to change their name and legal status, as well as changing their dress and appearance.
Patients need to live in their chosen gender in all aspects of their life; with friends and family and at work.
During this time, they would be given hormone treatment.
STAGE 3 - HORMONE TREATMENT
For women becoming men, the androgens they are injected with produce a deeper voice, more hair on the body, clitoral enlargement and the lightening and eventual ceasing of periods.
In men becoming women, oestrogen treatment triggers the development of breast tissue and a more feminine figure.
However, it does not change the timbre of voice or affect hair growth, so patients need to learn to speak differently and have electrolysis or laser treatment.
STAGE 4 - SURGERY
Not every gender reassignment patient will have surgery.
And those who do cannot have it straight away.
Dr Barrett said: "A patient would have to live as a man for at least a year before they were considered for a bilateral mastectomy [the removal of both breasts]."
They would also have their womb and ovaries removed, as the male hormone treatment could increase their risk of cancer.
Both men becoming women and women becoming men would have to wait at least two years before they could have genital surgery.
Creating female genitalia is a highly successful operation, according to Dr Barrett, who said many partners of people who have had the procedure could not tell they had once been a man.
But he said creating male genitalia was more complicated.
"There are lots of potential complications, and you need to take material from another part of the patient's body.
"Only around a third of patients want to have a penis constructed."