There are movies that deploy a certain sort of satirical pessimism, whose paradoxical function is to lend weight and credibility to whatever bittersweet glimmers of hope are finally offered. But this isn’t what happens in Saint Frances, a very moving film with complex, pertinent things to say about sexual politics, written by and starring Kelly O’Sullivan. It has an easy swing and an unapologetic, uncompromised idealism, but refuses the obvious narrative beats, particularly on the subject of pregnancy.

O’Sullivan plays Bridget, a woman in her early 30s who worries life is passing her by and, on a whim, sleeps with Jace (Max Lipchitz) a younger guy she meets at a party, who turns out to be really nice. And just as Bridget gets a nannying job for two women, Maya (Charin Alvarez) and Annie (Lily Mojekwu), looking after their infant daughter Frances (Ramona Edith-Williams), she discovers that she’s pregnant from her relationship with Jace. Unsure whether he’s the one, Bridget makes a calm and rational decision. This moment is arrived at without any great storm clouds of unhappiness, as it might be in a sterner “social issue” movie, and Bridget carries on with her life, which involves increasing intimacy with Frances and her formidable, difficult parents – and we are left to ponder the symbolism of this parallel.

She even has a fling with a conceited musician, because she doesn’t consider herself exclusive with Jace. But her original decision has left her with physical issues that cause repeated moments of embarrassment and mortification, particularly with men whose squeamishness on the subject is brutally revealed. It is the women who are understanding, and Frances who – perhaps – works a saintly miracle.

This is a film with a hopeful message about people, and their ability and willingness to learn – and to get along.

• Saint Frances is out in the UK on 24 July.