Guess How Much I Love You? is a work of pure compassion

So often, theatre rewards the big, bright, bold and fizzy: the play that shouts its ideas for all to hear; that flaps and waves so those in the gods can see. Productions live exuberantly, because they are mortal and their lifespan is short. Then something small and tender and well nurtured arrives, without fanfare, and does something with honesty that cannot be achieved with the most ingenious of tricks.

“Profound, they said,” the cry of an expectant mother, pacing in baggy nightie around a dingy bedroom. “The most profound example she had ever seen.” The woman in Luke Norris’s searing new work – starring two actors of great instinct, Rosie Sheehy and Robert Aramayo, and opening the Royal Court’s 70th anniversary season – is unnamed, like her unborn baby, though words are her gift: they come faster than her brain can make sense of them or what is happening inside her; what’s more, she’s misplaced her pamphlet.

She has just returned home with her husband after their 20-week scan. In the ultrasound room, awaiting the results, they played 20 questions: “alive or dead?” Now they must answer an unbearable yes or no: your baby has a 50/50 chance of survival, and in the best case, a life with complex needs and 24/7 care. Do you take the risk?

In some ways, this is simple subject matter: a moral dilemma and its brutal aftermath. An impossible Sophie’s choice, the woman realises: “What happens next is I’m a death camp.” The man wants to “park it”, and who will blame him? Guilt, grief, religion, a life-affirming love that clings, with frigid fingers, as the soul is ripped from it: these are familiar themes – the big ones. But played out in small, softly lit rooms (maternity unit, reception, a bathroom in the powdery palette of the powdery peach and sage of a children’s nursery) they are grounded in context. The husband plays their favourite song through an iPhone speaker, the wife orders Wagamama from her hospital bed: “Feels weird me sitting here with chopsticks.”

Dramatics are pleasingly undercut by sharp dark wit (there is a gruesome spin on the phrase “rotten breath”); sincerity by savagery (“Your face fills me with dread”). Push and pull; light and shade: a warm pregnancy glow surrounds the parents-in-waiting until harsh strip lighting ruptures it, along with the dream the father had of the three of them – a son with bright red hair – playing on the beach. The woman asks the midwife (Lena Kaur) to shut the curtains against a warm May day: “The sun doesn’t make any sense.”

There is a moment when the good-natured husband (played by Aramayo with a forced smile and persistent gentleness) laughs, desperate and incredulous, at the horror of things, which rings truer than the references to Hamlet. The script recognises this: the man is a sensitive, literary being, quoting Yeats and Shakespeare, yet it is a children’s story – Guess How Much I Love You? – that floors him. His evangelical optimism curdles with the bile in her belly, which erupts – in insults, howls, a jet of saliva – before settling once more. What shall we say of Sheehy? That she commands as much attention when unleashed by full-body grief. That her emotions lie so close to the surface we can see her cheeks flash red. A special talent, ferocious and funny, tragicomedy incarnate: directed by Jeremy Herrin, who led Denise Gough in a career-changing role in People, Places and Things, Sheehy can add to her credits (The Brightening Air, Machinal) another astonishing feat.

“Grow up, grow up, grow up,” she commands her husband, who is not, we suspect, the intended recipient. She delivers the line like an incantation, wishful yet powerless. Will the couple give up or will they try again? When at last they make it to the beach, the sun is a full circle, and the rocks rise up, like bumps on the horizon.

Guess How Much I Love You? is at Jerwood Theatre Downstairs, Royal Court, London SW1; until 21 February