The four words in this title are the four possible replies to bureaucratic tick-box questions about the frequency of your various sexual experiences. A young woman here must answer them, before she is allowed to have an abortion. However rigid and blandly routine it seems, the four-part answer grid is cleverly designed to get information about vulnerability: it is so easy instinctively and evasively to deny a difficult question structured as a yes/no, but much harder to check the “never” box, when “rarely”, “sometimes” and “always” are coolly offered as equivalently non-judgmental options.

The lead character in Eliza Hittman’s tough, realist drama is confronted with this central, four-part inquisition about her life in one brilliantly controlled, enigmatic scene. Theoretically, it is just a bit of form-filling that doesn’t appear to promise any real revelation to the audience. Yet it does just that, delivering a penny-drop moment of realisation. Or perhaps it’s more of an ambiguous hint and all the more disquieting for that.

The story is about a 17-year-old called Autumn, played by newcomer Sidney Flanigan, who is pregnant, and lives in a town in rural Pennsylvania where there is no question of being allowed a termination and no question of confiding in her family. After a tense encounter with an anti-abortion counsellor at her local clinic, Autumn gets help from her worldly-wise cousin, Skylar (Talia Ryder), who works alongside her at a supermarket checkout. Skylar scrapes the cash together so that she and Autumn can covertly get the bus together to New York where Autumn can get an abortion without anyone at home knowing. Their journey accumulates gradually in almost wordless tension and fear.

Never Rarely Sometimes Always is in some ways part of a tradition going back to Midnight Cowboy: the uncaring big-city bus station as the arena of poverty, vagrancy and the market forces of sexual exploitation. When Autumn and Skylar arrive in New York, absence of money is like an aura that pulses off them. You can feel the grit and chill of everything they touch as, without much of a clue, they ask for directions, and look like easy targets for predators. Hittman shrewdly emphasises the clumsy misery of Autumn having to pull her wheelie suitcase around the sidewalks, clunking up and down subway steps – all the discomfort of tourism and none of the pleasure. In fact, Autumn and Skylar have already been befriended on the bus by Jasper (Théodore Pellerin), a boy who seems friendly enough, though it is Skylar who reveals herself as the most wised-up of the three of them.

This narrative doesn’t accentuate abortion as an issue in ways you might expect, and abortionists and abortion are not freighted with obvious meaning in the way they are in, say, Cristian Mungiu’s 4 Months, 3 Weeks and 2 Days or Mike Leigh’s Vera Drake – movies set in very different eras, but perhaps not so different as all that, given the anti-abortion rhetoric so current in the US today. At first glance, the film it resembled for me is Juno, the film from screenwriter Diablo Cody, about a teenage girl played by Ellen Page who gets pregnant with her very first sexual experience.

That was a funny, articulate picture, but this is very different, and not merely because Juno does not, as it happens, want an abortion. Autumn has no intention of opening up to Skylar, or to us, about how she feels about being pregnant, or about a future in which she will no longer be pregnant, but in which all her existing problems will remain, having doubtless got worse. She remains almost impassive throughout, although there is a gripping opening scene at a school concert where she reveals her feelings in code by singing a version of the Exciters’ 60s hit He’s Got the Power. Autumn is jeeringly heckled by a guy in the audience who knows the truth (or heard some gossip) and, in an atmosphere of excruciating embarrassment, Autumn has to finish the song as best she can.

It is Skylar who saves Autumn, and their relationship is at the heart of the film; it develops enigmatically, and in near-silence. By the end, you can feel Skylar wanting Autumn to say something, anything – such as thank you, maybe. Hittman leaves Autumn’s personal life opaque, and she discloses her own feelings in more singing: her desolate karaoke version of a Gerry and the Pacemakers song: “The night’s the time for all your tears, / Your heart may be broken tonight, / But tomorrow in the morning light, / Don’t let the sun catch you cryin’…” Music is where the film’s emotional meaning is unveiled.

• Never Rarely Sometimes Always is available on digital platforms to buy from 13 May and to rent from 27 May.