It’s hard to perform a somersault at 36 weeks pregnant. Towards the end of his debut feature film, Jason Barker is swimming in the London Fields lido in east London, a short walk from the flat he shares with his partner, Tracey. The screen is rinsed blue. Barker dances, makes a star. And then, very slowly, he turns full height in the water, his Hawaiian swim shorts flapping, his stomach a perfect, firm dome.

This is the viewer’s first sight of Barker’s pregnant belly in A Deal With the Universe, which premieres at the BFI Flare festival next week. And after seven years in which he and Tracey tried to conceive, it is a moment of pure levity and joy. “That swimming stuff that you see?” he says. “It felt like the first time I could ever say, ‘Yeah! I actually like this body. Love it. It’s brilliant.’”

Barker was born female. He transitioned roughly 20 years ago, at 26, soon after he met Tracey – though, as Barker says, before and after don’t really work in this story. The process of transitioning was gradual, “without hard edges”. The two of them hoped to start a family, but after a few years of Tracey trying to conceive with her own eggs, in 2003 they resorted to “plan B”. Tracey would be impregnated; Barker, who had undergone chest surgery but kept his ovaries, would supply the eggs. He bought a new camera to document it. Soon they would have a baby – and a film.

So Barker stopped taking testosterone. He delayed an appointment to discuss a hysterectomy. Well, it was just a short film. Not too disruptive. But the filming went on and on – and Barker ended up telling a very different story to the one he planned. The pregnancy he chronicled was not Tracey’s, but his own. And it changed his sense of who he was.

Pregnancy among transgender men is increasingly common. Sally Hines, a professor at the University of Leeds who is leading a three-year research project into the subject, says: “In the UK, if you look at how many people are accessing blogs and online forums and support groups, asking about healthcare because they are pregnant, or young guys thinking about the future … There is lots of anecdotal evidence that more people are doing it. When something becomes visible, more people think it’s possible.”

But 10 years after Thomas Beatie, from Hawaii, made headlines with his combination of beard and baby bump – the first publicised case of a legal male, in a traditional marriage to a woman, to give birth – the data remains scant. In Australia, 54 people who identified as men gave birth in 2014, according to Medicare statistics. In the UK, the Office for National Statistics collects no information on the gender of the birthing parent; neither does the NHS. Last year, British newspapers including the Sun and the Independent hailed Hayden Cross as “Britain’s first pregnant man”. Soon afterwards, they had to hail another man, Scott Parker, after he got in touch to say that he had given birth a few months earlier.

“There have been about six first pregnant men,” Barker notes wryly. His son turns eight this year.

And yet the idea of transmasculine pregnancy as a novelty holds sway. Each birth is greeted as the first. It is perennially surprising, and I wonder if this is because it is conceived by cis people as a double-edged contradiction that undermines both the common conception of pregnancy as inherently female and the sense of a completed transition – as if a trans man carrying a child constitutes a sort of U-turn. But, as Hines says, “Transition is not a straightforward A to B … Pregnancy is not an interruption, just another part of a long and complex journey.”

Barker, whose son is now seven. Photograph: David Levene/The Guardian

Barker’s film begins with him prizing a state of heightened masculinity. When Tracey can’t conceive, he longs for “a penis and testicles”. He strikes muscular poses while washing the windows of their caravan, and the thought of his eggs entering her makes him feel like “cock of the walk”. He says it in a way that emphasises the pun. But, as the film progresses, a subtler story emerges.

“I had this fantasy picture,” he says. “I thought, I’ll have a baby, and that night I’ll go and have a pint, and about two weeks after that I’ll start on the testosterone again … Job done.” Pregnancy was a transient state, “a strange bit of my life”, after which normality would be restored.

But, pretty quickly, the film begins to transmit more mixed messages. As soon as he is pregnant, Jason appears in a pair of denim dungarees, that classic of 1970s maternitywear. In labour, he looks forlorn in a cerise nightie with a cute animal motif. He laughs when I ask why, in pregnancy, he resorted to these conventional cues for femininity. “You just grow so massive and you’ve got nothing!” he says. Retailers of pregnancy clothing aren’t exactly teeming with options for trans men. Tracey went to New Look and bought a load of maternity trousers, but even the combats were embroidered with flowers. She had to get a needle and unpick them.

In pregnancy, Barker mostly passed “as a fat bloke”. No one offered him a seat on the bus. No one batted an eyelid when, dressed in jeans and a cardie, he walked along the canal towpath to Nando’s two days before giving birth. He was both in plain sight and, owing to the relative rarity of pregnant men, hidden. In an antenatal class, when the teacher instructed all the pregnant folks to feel their hips, and he obeyed, the man beside him gave a nudge and said: “I don’t think we have to bother, mate!”

For as long as he could remember, Barker had had a body that didn’t fit. “And now you’re here,” he tells his son in the film, “and I can’t think like that any more.”

It would be easy to imagine pregnancy as a time of heightened gender dysphoria for trans men. “I had expected most people to have more dissonance with their body during pregnancy,” says Alexis Hoffkling, a researcher and medical student at the University of California, San Francisco who is trans herself. A few years ago, she started interviewing trans men who had been pregnant and found that while “some had a lot harder time with their bodies … [others] felt empowered”. Some found it masculinising. “They were more like a fat dude than they had ever felt before. As their body got bigger, they felt stronger.”

When Barker began to piece together the 25 hours of tape he had recorded over eight years of trying to start a family, a worry began to form. “The proper story,” he thought, “would be that somebody keeps their gender identity regardless. ‘I’m a man and I’m pregnant but I’m still a man, and this is a man’s pregnant tummy.’ But for me, it felt really different.”

Barker says he is naturally a very binary person. “I’ve been ever so serious about gender in my life. That it’s this thing you have to be fully committed to. Because my generation of trans people had to be fully committed in order to access treatment.” It took a while, but slowly he began to let go of his self-interrogation, what he calls “the whole pregnant man thing”. He stops to think. The closest comparison, he says, is that being pregnant was like watching Mo Farah run. “He is so graceful. He’s not having to go, ‘I’m trying to run!’ like the rest of us. And that’s how it felt for me: ‘Wow. I’m just doing this.’ It felt joyous and amazing and brilliant.”

In the same way that Barker would always stand up for his friends against transphobic strangers, now he felt compelled to protect his pregnant body against his own sense of incongruence. “I would defend that body. That body is a beautiful thing because of what it’s doing and what it’s done,” he says. The body was “all about my kid”.

So, in a way, it was a selfless body? It didn’t feel right when Barker was its sole occupant, but when it acquired another, an other, it became a better fit? I wonder if Barker felt less male when pregnant. But he says only: “Honestly, I had a really lovely time.”

“I’m going to ask you a very personal question,” he says, leaning forward. “When people talk about getting broody again, it’s pretty ick, isn’t it? It’s icky because none of us likes to think we are ruled by our hormones?”

That’s true, I say. But, speaking personally, I did get broody again.

“Yes, so did I!” he exclaims, delighted. He and Tracey knew that they wouldn’t try for another child, because it had taken them a decade to conceive and they didn’t want to lose their son’s infancy in endless rounds of IVF. But for a long time after the birth, Barker lived with a sort of second, shadow baby.

“I’d have these fantasies that somehow, a few months later, they’d say, ‘Just a minute! We think there’s a twin in there!’ I’d think about it all the time, that I was somehow accidentally pregnant and nobody knew. ‘Kangaroos do it,’ I’d think. Would there possibly be a way? They’d say, ‘We don’t know how it’s happened but it’s like your body stored it.’ And I’d be, ‘Well, there you go! A miracle!,’” he says.

He never had the hysterectomy. He hasn’t taken testosterone in 15 years, since he and Tracey embarked on their plan B.

Barker and his partner Tracey. Photograph: Sara Davidmann/A Deal With the Universe/BFI

There is, as yet, no guidebook to pregnancy for trans men, though Barker’s film will fortify others who wish to follow in his footsteps. There is a memoir, Where’s the Mother? Stories from a Transgender Dad, by Trevor MacDonald, who lives in Manitoba, Canada, and who carried his own children, now three and seven. MacDonald founded a Facebook group on birthing and breast- or chestfeeding for men.

The questions that come up repeatedly are practical ones. What is the impact of testosterone on a trans man’s chances of conceiving? (Barker took it for three years.) How does chest surgery affect lactation? (This subject is off-limits for Barker, but MacDonald fed his own children and became the first openly trans volunteer at La Leche League, the breastfeeding support group, after they initially told him it was “inappropriate” for them to help him. From his own research conversations with trans men, he knows that some found nursing reduced their experience of gender dysphoria around their breasts: “It seemed to have to do with those body parts serving a purpose that they otherwise didn’t,” he says.)

Once, in hospital, the nurses called Barker ‘Mum’. But after racking his brains, that is the single misstep he can recall. MacDonald says he is amazed that Barker had such a smooth experience with his healthcare providers. There are plenty of stories of those who don’t – specimen bottles routinely given to thin female partners instead of the pregnant man, and so on.

Registering the birth was a hurdle, though. Barker had no choice but to officially be his son’s mother. As well as his name, he included his birth name under the designation “AKA”, because he dreaded “some kind of clerical error that would make the baby not mine”. That sounds “completely paranoid”, he says, but growing up with section 28 had given him the idea of “not thinking that you deserve to be spoken about, do certain jobs or have certain things”. And those things included, presumably, a child.

Mostly, though, parenthood has been free of administrative challenges. In fact, “Being out and about as a dad with a small baby attracted more attention than being a ‘pregnant man’ did!” he says. Barker is a writer, but he is also his son’s prime carer. “I remember him crying on a bus and a woman shouted, ‘Not as easy as you thought, was it?’ Also I once shocked the whole Stay and Play when I told them that my partner had gone back to work two weeks after the baby was born.”

Their little boy has grown up with an understanding of his family, how he came about. Ever since he was born, they have told it to him almost like a bedtime story. (He has heard it so often, he sometimes rebukes his dad for “boasting about being trans”.) Discussing Harry Potter one day, Barker’s son wanted to know which of the characters Barker would be if he could choose anyone. “Hagrid,” Barker replied. In the films, the hirsute, giant gamekeeper is played by Robbie Coltrane. “Well, you’ve already got the beard,” his son said, appreciatively.

A film needs an arc, of course – to end somewhere other than where it started. Barker’s worry about this, when he began to edit the footage last year, was, “My God, will somebody think I’m cured [of being trans]? It’s a horrible thought,” he says. It’s all right for Tracey, his indomitable partner, whose eyes continue to sparkle even through a mastectomy for breast cancer. She didn’t need an arc. She could just be brilliant all the way.

“I think my arc,” he says, “is going from somebody who thinks being an ordinary man is the best thing you can be to somebody who sees a different way of being. To a certain extent it’s about femininity,” he says, tapping the table as if he’s put his finger on it. “I’d pushed [it] away from a really young age, and I think it’s about bringing some of that back. And you realise how undervalued this work is … And it does make you think, ‘What was I pushing away? What was I scared of?’

“It’s about vulnerability, I think,” he says, and it is a surprise to hear him say it, because the quality he most wants to surface in the film is resilience. Of course, the two go together, and Barker’s story is about both of those things, and the personal regrowth that can come from giving birth. He not only challenged boundaries in the world around him, but in his own understanding of himself.

A Deal with the Universe is at BFI Flare: London LGBTQ+ film festival on 26 March at BFI Southbank