On demand: "Seahorse: The Dad Who Gave Birth"

Changing times. Seahorse, the latest film by the British documentarist Jeanie Finlay, is on some basic level your common-or-garden pregnancy narrative, one that begins with great expectations and ends in new life. Yet the route it takes to get from one state to another is somewhat complicated. Freddy McConnell, the subject Finlay encounters getting broody on the seafront in his hometown of Deal, is a trans man, born with female sex organs, who's elected to abandon the course of testosterone he's been put on in order to push through with his life goal of becoming pregnant, and a parent. If that wasn't enough for retired Home Counties colonels to get their head and pipes around, Freddy's partner CJ is a Trinidadian immigrant who self-identifies as non-binary and gay. Everything about this Guardian Films production would seem custom-made to set Daily Mail readers to exploding in fits of spluttering rage. There are shapeshifters among us? Attempting to procreate? In the Garden of England, of all places? Stiff letters have been written to local MPs for less.

The film counteracts the complication, and any attendant consternation, via the straight-ahead simplicity of its approach. As her 2011 breakthrough Sound It Out indicated, Finlay ranks as one of our most empathetic observers: here, she sets us down gently next to a fellow human being going through a potentially fraught moment, and invites us to watch and perhaps learn. Seahorse is at root a study of a man trying to do two things simultaneously, which - as seasoned watchers of the world will be aware - is a tricky thing for a man to do, whatever the configuration of his internal organs. Finlay finds Freddy striving to remain female enough to bring a child into this world while still being the manly man he's wanted to be ever since his teenage self Pritt-Sticked images of Mel Gibson into a scrapbook. (You begin to understand his confusion.) His plight yields plentiful ironies, not least when Freddy sighs "if men had to do this [carry a child], you'd never hear the end of it", but also poignant moments of introspection: Freddy lets something slip when he confesses he hasn't spoken to cis males about his pregnancy, lest they think him "less of a man", not seeming to realise that this gestation period requires the strength of both sexes combined. The gender essentialism Freddy lapses into would be exhausting even without the hormonal ups-and-downs of the fertility process: the whole movie plays like an exhortation to just be who you are, and not worry too much about what others make of you, or about living up to the ideal in your head - though I'll concede this is obviously easier for a fortysomething cis bloke to type than for a younger trans man to put into practice.

At any rate, if it was a miracle that Finlay landed on this semi-miraculous story in the first place, it's as nothing compared to the exemplary sensitivity she displays as she inserts herself into Harley Street examination rooms, or adjacent to Freddy on the couch as someone he thought was an essential part of his support network recedes into the distance. She's always there, part-director, part-midwife, nudging both her subject and the viewer past any complications that may arise; you can't fail to notice the very great trust Finlay fostered with a subject who palpably fears ridicule, and most often seems pregnant with doubt, especially when he dips into the comments below online news stories about others in the same position. (You can imagine.) Another filmmaker might have come back with a more sensational retelling of this story, keeping an opportunistic eye on the heady heights of a Channel Five primetime slot. Finlay, for her part, contextualises Freddy's progress within an everyday reality of shonky washing machines and nappies from Aldi's Mamia line; she leaves in time and space to get our heads around anything we need to get our heads around; and it undeniably helps her cause that, one hormone-induced strop aside, Freddy presents as a total sweetheart who deserves whatever happiness he chooses for himself. It's a sign of how rapidly the world is changing that this exact premise was treated as an utterly inconceivable joke by an Arnold Schwarzenegger movie as recently as 25 years ago. (Did the young Freddy see it? Or was he too busy swooning over the same year's Maverick?) In Finlay's supremely careful hands, this story doesn't seem anything like as preposterous as it did there, or indeed beyond the reach of our understanding - just a couple more square feet of development in life's increasingly rich tapestry.

Seahorse: The Dad Who Gave Birth is now streaming on the BBC iPlayer.