"Why can't you fool an aborted baby? Because it wasn't born yesterday." If you find that so-called joke hard to take, you might also have a problem watching 22 year-old New York playwright Ruby Rae Spiegel's locker room drama set on a contemporary college campus in Florida.

The speaker is desperate, sardonic Amy (Milly Thomas), who is enjoining her skinny friend Ester (Aisha Fabienne Ross) to punctuate her training sessions in the offstage pool with a punctuation of her own pregnancy. Amy doesn't want the baby, and even more doesn't want her mom to know about it, so offers Ester her stomach as a punch bag.

There are no punches pulled, either, in the scatological dialogue that accompanies this gruesome extended ritual, covering Halloween dates with boys during menstruation – intimacy with Wolverine becomes tricky when blood seeps down your Bugs Bunny outfit – the trouble with pornography (Amy taunts Ester with a copy of Hustler), having sex on a trampoline, self-harming, zombies, drugs, Harry Potter and the nutritional value of bananas.

The awful thing about the play, which is smartly directed by Hannah Hauer-King and presented with a cool technical efficiency – design by Anna Reid, lighting by Joe Price, sound by Nathan Klein – is that it rings horribly true. There's absolutely nothing glib or gratuitous about the problem and symptoms it anatomises, and the acting is both well-judged and utterly authentic.

Amy and Ester – occasionally joined by a swimsuit sister Reba (Charlotte Hamblin) who is no less explicit in her casual conversation – have a big girl/little girl, subliminally sexual relationship going on, too. But as Amy's dilemma deepens, and she presses Ester both to her purpose and her tyranny, so Ester begins to emerge like a butterfly from its chrysalis, seemingly liberated by her enslavement.

The play, which is as taut and tight as the swimmers' tummies (except Amy's, of course, which Ester discovers has the start of a little turtle lump in it), takes just 100 minutes, but covers a lot of ground. There's even an extremely well-written scene where Ester goes to visit a putative boyfriend Victor (Dan Cohen) to share her misgivings, spots and possibly her lips; they have to snuggle down in the hallway while his room-mate makes out on the other side of the door.

Ester's coming through – more than a coming out – is touchingly charted and calibrated by Fabienne Ross, while Amy's view softens somewhat in Thomas's exquisite and compelling performance, and the play performs a pleasing loop off the diving board. In a penultimate scene of difficult-to-watch poignancy, the arrival of the phlegmatic janitor (Mark Keegan) with mop and bucket suggests these extraordinary events are an everyday occurrence.

Dry Land runs at the Jermyn Street Theatre until 21 November.