The testosterone injections reduced the participants’ levels of Anti-Mullerian Hormone (AMH), which signals how many eggs a woman has.
But for the first year the trans men still had enough of the hormone to signal they were fertile enough to get pregnant.
It is not clear what their AMH readings were, but between 1ng/ml and 3ng/ml is considered normal, according to the Reproductive Medicine Associates of New Jersey.
Less than 0.6ng/ml is considered ‘low’ and could impact fertility.
Only men who haven’t had sex-change surgery would still be able to get pregnant, and they’d have to conceive the normal way – through vaginal sex with a biological man.
The researchers admit their study was small, with useable data from only 32 people, but said scientific studies of this kind are rare.
They revealed their findings at the Endocrine Society’s annual conference in New Orleans on Saturday.
The British Government doesn’t know how many transgender people are in the UK, but estimates the figure is between 200,000 and 500,000 – up to 0.75 per cent of the population.
Research published last year revealed men changing into women may also have a hard time getting hormones under control using medicine alone.
Boston University scientists found most trans women couldn’t reduce their testosterone to female levels with drugs.
A study of 98 trans women, published in the journal Endocrine Practice, found only a quarter had managed to get their testosterone levels into a female range.
The same proportion hadn’t been managed any ‘signficant suppression’ of their male hormones at all.
Wyley Simpson, 28, was shocked to find out he was pregnant in February 2018 after living as a man since he was 21.
Mr Simpson, from Texas, began taking testosterone therapy in 2012 and had his breasts removed in 2013, but still has a vagina and female reproductive organs.
He became pregnant after having unprotected vaginal sex with his fiancé, Stephen Gaeth, 28, and gave birth to Rowan via c-section in September last year.
Mr Simpson claims doctors told him he wouldn’t be able to fall pregnant because of the therapy and he had stopped having periods.
Although they initially did not feel ready to become parents, the couple followed through with the pregnancy.
He said: ‘Rowan is amazing, he made all of the mental and physical pain completely worth it.
‘Now myself and Stephan are just revelling in fatherhood and can’t wait to continue our journey as a family.’