Becoming a mother reveals a woman's capacity for numerous things: virtue, self-sacrifice, anger, foolishness, love. Some of these qualities will never before have been tested – she may not even have known that she possessed them. Some of them will take their shape exactly from what she was offered by her own mother, though she may not remember being offered them. And some – anger is one – will find forms of their own, of which she feels herself to be the only progenitor.

Rebecca Asher's Shattered: Modern Motherhood and the Illusion of Equality wishes to transform this specific, personal anger into something general and wide-ranging, and hence to put it at a certain distance from herself. Had she not become a mother herself, she would not have written it; she may even have remained unaware that there was anything in motherhood to become angry about. The book is an attempt to reverse this flow of information, to find objective "evidence" for what she has discovered through private experience, that after the birth of her child a woman's equality is shown to be not the great evolutionary transformation of the modern era, but a kind of temporary, easily revoked passport into the world of men. In Shattered, the emotional becomes the factual and statistical; the private is refracted through the stories of others, elicited in interviews with parents up and down the country and abroad.

Why does a woman become angry when she has a baby? One answer is: because she has lost the power of autonomy and free will in her own life. From the first moment of her pregnancy, a woman finds herself subject to forces over which she has no control, not least those of the body itself. This subjection applies equally to the unknown and the known: she is her body's subject, her doctor's subject, her baby's subject, and in this biological work she has undertaken she becomes society's and history's subject too. But where she feels the subjection most is in the territories, whatever they are, that in her pre-maternal life she made her own. The threat to what made her herself, to what made her an individual: this is what the mother finds hardest to face down. Having been told all her life to value her individuality and pursue its aims, she encounters an outright contradiction, a betrayal – even among the very gatekeepers of her identity, her husband or colleagues or friends – in the requirement that she surrender it.

In fact, a woman is never more of an individual than in her response to the conformity of motherhood: it is her individuality, after all, that is being questioned. This is one of the things that makes the culture of motherhood so difficult to grapple with. Motherhood may look like a genre, but in reality it is an infinitesimal network of secret pacts and compromises, of private bargainings with the self. Beginning with pregnancy and birth, it is taxing in its solitude, as well as taxing of solitude; it takes away all the profit in being alone, and leaves mere loneliness. The new mother – and not just the new mother – is lonely, and yet never on her own. And it is, again, as individuals, not as socio-economic groups or classes of people, that we respond to these difficulties. One mother, being angry, might slap her child in the supermarket; another might write a book about her anger. A teenage girl and a fortysomething female chief executive might both, on the other hand, for reasons of personality, enjoy being given something to love and look after. A highly educated woman and a severely deprived woman might have more in common with each other's emotional responses to motherhood than with those of their friends.

Rebecca Asher is not wrong to see a general hazard in the adaptation of culturally equal, post-feminist woman to the time-honoured injustices of family life, but formulating a cri de coeur out of it is a delicate matter. Her own indignation has its genesis in the fact that after the birth of their son, the equality that she and her husband, both working professionals, believed they shared was proved to be artificial: with a newborn baby in the house, he was free to resume his old life and return to work, while she was not. Where, then, had this impression of equality come from? And who was responsible for its disappearance? Well, like many intelligent women of her age, no one ever told her that she wasn't equal: it was from outside that the impression came, from the attainments she was free to pursue, the liberties that were available to her, the achievements and jobs she was able to acquire. And no one ever told her husband that he was fettered by his biological destiny, that with the birth of his child his whole life would have to change. So while the old idea of woman had certainly been done away with, no one had thought to put anything in its place. The baby comes and everyone panics, looking for the woman who's going to take care of it, and like the doctor of philosophy summoned over the PA to attend a medical emergency, there's a case of mistaken identity. "Mother" and "woman" are at this stage still just words, but it's a crisis so she'll have to do. A year, three years, five years later she's become a mother, a woman, through hour after hour of experience – and now she really is different, different from her husband, and different from the person she was.

"The expectation that the mother is primarily responsible for the children and family home disfigures our relationships," Asher writes. It might have been better if she had said it had disfigured her relationships, for true as it might be, put so generally it is hard to know exactly what kind of truth it is. Among other things it is the truth of hindsight, of after-knowledge: it is knowledge born of experience. The lesson here is not about motherhood so much perhaps as about womanhood, womanhood not as something granted – and removed, should the necessity arise – by the cultural conditions, but as something personally realised. If women experience equality only from the outside, in the form of their freedom to pursue male values, the intransigence of biological identity comes as a surprise. It is as women, not as mothers, that we should watch more closely, listen harder, think more deeply. The modern woman arrives in motherhood like a shellshocked tourist without a guidebook in a foreign land, and Shattered rails against the brutality of the transfer, the injustice of the new conditions. It could be said that this is the real proof of inequality, this vulnerability to the status quo; and that motherhood is a lesson in responsibility, learned too late.

Rachel Cusk is the author of A Life's Work: On Becoming a Mother