An elderly psychic in Hilary Mantel's novel Beyond Black is fond of asking her clients: "Have you known the joys of motherhood, dear?"

It's not a phrase you hear nowadays. Motherhood is a terribly serious business, with terribly serious consequences: one misstep can doom your baby to years of therapy later on. Or worse, as one parenting manual warns: "Brain scans of impulsive murderers showed that under stress, [they] had lots of activity in the lower brain and very little in the higher brain . . . This can be the legacy of a childhood where children have not been helped to work through feelings of rage and distress by parents." Murderers! No wonder new mothers aren't feeling much joy.

Claire Dederer had her baby about 10 years ago in north Seattle, a liberal enclave of well-educated white folks and free spirits that trumps any British neighbourhood for right-on-ness. Attachment parenting – a philosophy that advocates breastfeeding on demand, co-sleeping and generally slinging your offspring about your person, like a monkey – was considered optimal, and, as Dederer reports: "For the kind of mothers we were, optimal meant mandatory." That is, if the authorities recommend breastfeeding for a year, you carry on for several; if you have a second baby before your first is ready to stop co-sleeping, you buy a bigger bed to get all four of you in. Dederer finds herself racked with anxiety – and baby-induced backache. This book tells how her love affair with yoga changed her way of thinking and eventually her way of life.

A wry eyebrow may be raised at this point. Not another westerner-healed-by-eastern-mysticism; not another journey to find oneself in the footsteps of Elizabeth "Eat, Pray, Love" Gilbert (whose endorsement graces the cover)? Thankfully, Dederer is too smart, too funny and too sceptical herself to follow that path. She skewers the hypocrisy of westerners who think they really, y'know, get yoga: "I had a feeling that doing yoga in a class, without knowing the philosophic and historical underpinnings, made me kind of a jerk." What's more, she's not very good at it. She can't get her head around the paradoxes in Patanjali's Yoga Sutras, and she can't do the splits. Nor can she concentrate, at first: even when trembling with pain in the difficult poses, she finds her mind drifting helplessly. (Did Patanjali ever catch himself wondering what to give the kids for dinner?)

But it's these drifts and digressions that lift Dederer's book above the rest of the mystical-journey crowd. She gives us the poignant story of her faltering marriage and, many years before, the spectacular collapse of her parents' relationship. In 1973, Dederer's mother left her husband for a hippie type 16 years his junior. Dederer links her mother's departure to the wave of feminist feeling that boosted American divorce rates between 1967 and 1977: "Men had always left . . . now the women were leaving, too." It was the age of Erica Jong and Marilyn French and, if you didn't like where you were, you could just go – a revolutionary idea for women brought up to believe, as Dederer says, that liberation would come only with marriage.

But what about their legacy? What happens, asks Dederer, when a generation of children grows up with parents who want to be free, and who think that freedom is movement? Those divorces may have liberated the mums, but they left the dismayed kids with funny ideas about responsibility and happiness. Is it possible to "just go" in a world where, when it comes to your children, optimal means mandatory?

All this has taken us a long way from downward dogs and sun salutations. But that's no bad thing; the book is at its best when Dederer's quest for personal enlightenment gives way to broader sociological musings. Indeed, the book might have benefited from more investigation into issues of post-feminist maternity, and less yoga-blog-speak ("Inhale. Engage your core. This is a moment of readiness"). There's also a lot of chatty stuff about issues such as choosing an ultra-liberal school for young Lucy. A good edit would have allowed Dederer to get more deeply under our skins.

She is an engaging, snappy writer, though, and her book is a pleasure. If the material is baggy and loose, it knits up well enough to form her point: that it's OK – perhaps essential – to suck at yoga, and to know you suck, because yoga doesn't make you strong, it reveals your deepest weaknesses. A mystical journey to suckiness: now that's a quest worth believing in.