London - A transgender woman has become the first in the world to successfully breastfeed her baby after hormone therapy.
The 30-year-old, who was born a man, exclusively fed her baby for six weeks, doctors report.
She then continued to breastfeed alongside using formula until the infant was six months old.
It is the first recorded case in which a transgender woman has been able to produce enough milk to support a child.
The woman, who has not been named, approached doctors in New York after her partner became pregnant.
She had received no surgery to transition from a man, but had been undergoing hormone therapy for some years and had already developed fully-grown breasts.
Writing in the Transgender Health journal, specialists from the Mount Sinai Centre for Transgender Medicine and Surgery said: "She explained that her partner was pregnant but not interested in breastfeeding, and that she hoped to take on the role of being the primary food source for her infant."
The woman was already using drugs to block her male testosterone hormones, and taking female hormones estradiol and progesterone.
Over the next three months she was given domperidone, a drug that boosts levels of prolactin, the hormone which naturally triggers milk production in women.
She was also given a breast pump to simulate a feeding child, in a bid to further boost milk production. Two weeks before the baby was due she was producing 8oz of breast milk a day.
The doctors wrote: "Three and a half months after she had started the regimen, the baby was born weighing 6lbs 13oz.
"The patient breastfed exclusively for six weeks…[and] the child’s growth, feeding, and bowel habits were developmentally appropriate.
"At six weeks, the patient began supplementing breastfeeding with 4–8 oz of Similac brand formula daily due to concerns about insufficient milk volume.
"At the time of this article submission, the baby is approaching six months old. The patient continues to breastfeed as a supplement to formula feeding."
The researchers added: "We believe that this is the first formal report in the medical literature of induced lactation in a transgender woman."
Dr Tamar Reisman, endocrinologist at Mount Sinai, told the Mail: "Hypothetically there is nothing stopping men from breastfeeding – the ability is inherently there.
"But the risk-benefit profile would be entirely different and there would be a lot for a man to consider as the patient here was already on long-term feminising hormones that contributed to the ability to produce milk. Although it is not entirely clear which ones were primarily responsible, it may be that the use of a breast pump alone would have been sufficient to increase prolactin, the hormone necessary to produce milk."