On a snowy night in 1910, a baby is born with the cord wrapped around her neck and, because the doctor is delayed by the weather, she is dead within minutes. But a page or so later, she's born again and this time the doctor makes it through the snow "just in the nick of time" and the baby lives.

Ursula enjoys a balmy, prewar, English country childhood, with brothers and sisters and servants and picnics and seaside outings. But the theme of drastically alternative outcomes continues. At five years old, playing in the waves with her big sister, Ursula slips in the undertow and drowns.

Yet again, we prepare to mourn her. But no, here it is all over again  the summer's day, the crucial, dangerous moment. And this time an amateur artist happens to have set up his easel close to where the children are playing and he sees Ursula go under and returns both girls "sopping wet and tearful" to their grateful mother.

Much of the (very considerable) pleasure of this almost deliriously inventive, sharply imagined and ultimately affecting novel lies in the almost spookily vivid atmosphere and pathos that Atkinson manages to extract from all this Groundhog Day repetition. The premise  so pregnant with narrative opportunity that you wonder why no novelist has explored it before  is simple. What if we had a chance to live life "again and again, until we finally did get it right?"

In the early pages, this preoccupation with "getting it right" inspires quite dizzying heights of narrative suspense. So, Bridget the maid goes off to London on Armistice Day to celebrate and mingle with the crowds and returns late that night and, regaling them all with her stories, unwittingly  and fatally  infects Ursula and her siblings with Spanish flu.

A few pages later, the rerun: eight-year-old Ursula, hearing Bridget come back from London and feeling "a great dread, as if something truly treacherous were about to happen" stays in bed and doesn't go downstairs to hear the stories. And she lives. Several versions later however, Ursula, now troubled by an increasingly powerful sense of deja vu, does everything she can to stop Bridget going to London, including pushing her down the stairs (it works, Bridget stays). And then, in an almost comically chilling coda, Bridget, having felt "hands shoving me in the back like the hands of a little ghost child" falls and sprains her ankle  only to hobble off to London (and her fate, their fate) regardless.

These virtually identical, yet subtly different versions of the same events feel both poignant and electric to read. Atkinson's knack for retelling  what to repeat, what to change, what to leave out  is satisfyingly faultless. Most of all, though, there's an odd exhilaration in the sheer number and the build-up  like one of those old childhood clapping songs that go faster and faster until you fall down, breathless and laughing.

Meanwhile, Ursula's continuing premonitions compel her parents to send her off to see a psychiatrist keen to talk reincarnation, but Atkinson wisely doesn't take this too far. Too much probing and explanation would deaden the novel's natural vivacity. Instead, we watch as the girl grows up to continue to grapple with circumstance: making choices, taking chances, getting it wrong and right, though, of course, never really all that right.

At 16, raped and pregnant by a friend of her brother's, she undergoes a backstreet abortion. But in a euphoric replay, she rebuffs the rapist and continues on, strong and undamaged. There's a grimly convincing episode where she endures  and escapes  a violent marriage. And a slightly less convincing one, where she ends up in prewar Berlin, married to a German and hobnobbing with Adolf and Eva.

In fact, if the novel has a downside, it's that the Berlin episode turns out to be crucial to the plot and, though I'll admit Atkinson wraps everything up with elan, I wasn't sure I really cared. And of course, if anything can happen  if people can die one minute and come back to life a couple of pages later  then catastrophes such as death lose a lot of their currency.

But Atkinson has done her research and then, as novelists should but rarely do, lit a rocket of her own under it. The scenes set in Blitz-stricken London will stay with me forever, especially the description of the dead man whose "body came apart like a Christmas cracker".

Her real strength, though, lies in her people  fathers, brothers, sisters, aunts. This novel is constantly acute and real and touching about familial and (especially) sibling love. Impressively  and unusually  some of her most engaging and best-realised characters are the good, kind, decent ones. Not only that but (a sure sign of heart in a novel) her dogs are great.

Best of all, Atkinson has written something that amounts to so much more than the sum of its (very many) parts. It almost seems to imply that there are new and mysterious things to feel and say about the nature of life and death, the passing of time, fate and possibility. When the baby Ursula, lying outdoors in her pram, sees winter come again and realises she "recognised it from the first time around", you catch your breath. And you don't really get it back until the very last page of this magnificently tender and humane novel.

Julie Myerson's new novel, The Quickening, is published on 28 March

Kate Atkinson's new novel is a marvel, a great big confidence trick  but one that invites the reader to take part in the deception. In fact, it is impossible to ignore it. Every time you attempt to lose yourself in the story of Ursula Todd, a child born in affluent and comparatively happy circumstances on 11 February 1910, it simply stops. If this sounds like the quick route to a short book, don't worry: the narrative starts again  and again and again  but each time it takes a different course, its details sometimes radically, sometimes marginally altered, its outcome utterly unpredictable. Atkinson's general rule is that things seem to get better with repetition, but this, her self-undermining novel seems to warn us, is a comfort that is by no means guaranteed, either.

She begins as she means to go on, and at the very beginning. (In fact, even this is not quite true: a brief prologue shows us Ursula in a Munich coffee shop in 1930, assassinating Hitler with her father's old service revolver.) At the start of the novel "proper", Sylvie Todd is giving birth to her third child, her situation given a fairytale atmosphere by the encroaching snow which also, alas, cuts her off from outside help in the form of Dr Fellowes or Mrs Haddock, the midwife. Ursula is stillborn, with the umbilical cord wrapped around her neck, her life unsaved for want of a pair of surgical scissors. Fortunately, though, she is allowed another go at the business of coming into being; in take two, Dr Fellowes makes it, cuts the cord and proceeds to his reward of a cold collation and some homemade piccalilli (it might be too fanciful to notice that even the piccalilli repeats).

Ursula's childhood is to be punctuated with such near-misses: the treacherous undertow of the Cornish sea, icy tiles during a rooftop escapade, the wildfire spread of Spanish flu. Each disaster is confirmed by variations on the phrase "darkness fell", and each new beginning heralded by the tabula rasa that snow brings. Ursula carries within her a vague, dimly apprehended sense of other, semi-lived lives, inexpressible except as impetuous actions  such as when she pushes a housemaid down the stairs to save her from a more terrible ending. That misdemeanour lands her in the office of a psychiatrist who introduces her, in kindly fashion, to the concept of reincarnation and to the roughly opposing theory of amor fati, particularly as espoused by Nietzsche: the acceptance, or even embrace, of one's fate, and the rejection of the idea that anything could, or should, have unfolded differently.

Amor fati is tough to take, of course, if you are a drowning child, or a battered wife, or a shell-shocked young man, or a terrified mother calling for your baby in the rubble of the blitz, all of whom and more besides make up the lives captured, however fleetingly, in Life After Life. It's equally tough if you are a novelist, and put in the powerful but invidious position of controlling what befalls your characters. Are their futures really written in their past? Can you tell what's going to happen to them simply from the way you started them off? Even sustaining your creative engagement could prove tricky: perhaps that's why one catastrophe is tagged with the exhausted words "Darkness, and so on" and why yet another recitation of Ursula's birth is reduced to a mere five lines.

The reader is similarly implicated in this continual manipulation of narrative tension and the suspension of disbelief. We want a story, but what kind of story do we want: something truthful or something soothing, something that ties up loose ends or something that casts us on to a tide of uncertainty, not only about what might happen, but about what already has? In Atkinson's model, we can have all of the above, but where does that leave us, with multiple tall tales clamouring for our attention?

Sometimes, it appears we are being offered a straight choice between happy and unhappy endings. On the one hand, there is Fox Corner, the Todd family home in what is still, although perhaps not for long, a wonderfully bucolic England. There are gin slings and tennis on the lawn and bees buzzing their "summer afternoon lullaby"; there is the reliable accumulation of children  Ursula is the third of five  and servants that are either touchingly steadfast or humorously difficult; there are beloved family dogs and treasured dolls and troublesome aunts whose bad behaviour can just about be absorbed.

Outside in the lane, however, lurks an evil-minded stranger, his story the more powerful for never being brought into the light; and sometimes intruders arrive under the cloak of friendship. When Ursula is molested, and then raped, by a pal of one of her brothers, her exile from Fox Corner begins; her subsequent pregnancy and illegal abortion give way to a lonely London life, solitary drinking and then, most awfully, to a violent husband who shuts her up in a mean little house in Wealdstone, far from her family.

Ursula's marriage to the vile Derek Oliphant  himself a constructor of false personal history  would never have happened if she had managed to evade her teenage abuser. In the next iteration, she does; and she is liberated once more, to plunge on to lives made perhaps even more divergent by the schism of the second world war. And the reader is perplexed once more: what to make of a character so chameleon-like that we can watch her excavating bomb sites on one page, stranded in a dystopian, war-torn Berlin on another and (in what admittedly requires the biggest leap of faith) being entertained by the Führer at Berchtesgaden on yet another?

This description of Atkinson's looping, metamorphosing narrative inevitably makes it sound tricksy, almost whimsical. Structurally, it is, but its ceaseless renewals are populated with pleasures that extend beyond the what-next variety. She captures well, for example, the traumatic shifts in British society  and does so precisely because she cuts directly from one war to the next, only later going back to fill in, partially, what happened in between. She demonstrates an extraordinary gift for capturing peril: the sections in which influenza tears through Fox Corner are truly menacing, and the descriptions of Ursula's work in a bombed-out London are masterpieces of the macabre ("'Be careful here, Mr Emslie,' she said over her shoulder, 'there's a baby, try to avoid it.'").

The texture of daily life is beautifully conveyed, particularly in its domestic details, which often verge on the queasily visceral. An ineptly poached egg is "a sickly jellyfish deposited on toast to die"; shortly after Sylvie's confinement, Mrs Glover, the crosspatch cook, "took a bowl of kidneys soaking in milk from the pantry and commenced removing the fatty white membrane, like a caul". On another occasion, she thumps slices of veal with a tenderiser, imagining "they're the heads of the Boche". But alongside these minutiae is set the author's fascination with the intricacies of large families, and in particular with sibling relationships.

The so-called family saga is, of course, where Atkinson's career as a novelist began, with the Whitbread-winning Behind the Scenes at the Museum, itself a story that refused to proceed in linear fashion, invoking the spirit of Tristram Shandy in its digressive portrayal of the life of Ruby Lennox. Neither book, of course, can really be contained by such a constricting label, just as Atkinson's four Jackson Brodie novels refuse to fit neatly into the genre marked crime. Behind the Scenes and Life After Life both co-opt the family  its evolution over time, its exponentially multiplying characters and storylines, its silences and gaps in communication  and use it to show how fiction works and what it might mean to us. But what makes Atkinson an exceptional writer  and this is her most ambitious and most gripping work to date  is that she does so with an emotional delicacy and understanding that transcend experiment or playfulness. Life After Life gives us a heroine whose fictional underpinning is permanently exposed, whose artificial status is never in doubt; and yet one who feels painfully, horribly real to us. How do you square that circle? You'd have to ask Kate Atkinson, but I doubt she would give you a straight answer.