A bright slice of sunlight moved slowly across the wall, and when it reached the lamb hiding behind the hedge, Olivia climbed out of bed and pushed her feet obediently into her small slippers, pink with rabbit faces and rabbit ears, and much coveted by Julia. None of the others bothered with their slippers, and now it was so hot that Rosemary couldn't even get them to wear shoes, but Olivia was a biddable child.

Rosemary, lying in her own bed, awake, but with limbs that she could barely move, as if the marrow in her bones had turned to lead piping, was at that very moment trying to devise a plan that would stop the other three from corrupting Olivia's good behavior. The new baby was making Rosemary feel sick, and she thought how wonderful it would be if Victor suddenly woke from his snore-laden sleep and said to her, "Can I get you something, dear?" and she would say, "Oh, yes, please, I would like some tea – no milk – and a slice of toast, lightly buttered, thank you, Victor." And pigs would fly.

If only she weren't so fertile. She couldn't take the pill because it gave her high blood pressure, she had tried a coil but it dislodged itself, and Victor saw using condoms as some kind of assault on his manhood. She was just his broodmare. The only good thing about being pregnant was that she didn't have to endure sex with Victor. She told him it was bad for the baby and he believed her because he knew nothing – nothing about babies or women or children, nothing about life. She had been a virgin when she married him and had returned from their one-week honeymoon in Wales in a state of shock. She should have walked away right there and then, of course, but Victor had already begun to drain her. Sometimes it felt as if he were feeding on her.

If she had had the energy she would have got up and crept through to the spare bedroom, the "guest" bedroom, and lain down on the hard single bed with its daisy-fresh white sheets anchored fast by tight hospital corners. The guest bedroom was like an air pocket in the house, its atmosphere not breathed by anyone else, its carpet not worn by careless feet. It didn't matter how many babies she had, she could go on dropping them like a cow, year af-^ ter year (although she would kill herself if she did), but not one of them would ever occupy the pristine space of the guest bedroom. It was clean, it was untouched, it was hers. The attic would be even better. She could have it floored and painted white and put in a trapdoor, then she could climb up there, pull up the trapdoor like a drawbridge, and no one would be able to find her. Rosemary imagined her family wandering from room to room, calling her name, and laughed. Victor grunted in his sleep. But then she thought of Olivia, roaming the house, unable to find her, and she felt fear, like a blow to her chest. She would have to take Olivia up to the attic with her.

Victor himself was in that kind place between waking and sleeping, a place untainted by the sour feelings of his everyday life, where he lived in a houseful of women who felt like strangers.

Olivia, thumb plugged snugly into her mouth and Blue Mouse clenched in the crook of her elbow, padded across the hallway to Julia and Amelia's bedroom and clambered in beside Julia. Julia was dreaming furiously. Her savage hair, plastered to her head, was wet with sweat and her lips moved constantly, muttering gibberish as she battled with some unseen monster. Julia was a heavy sleeper: she talked and walked in her sleep, she wrestled the bedclothes and woke up dramatically, staring wild-eyed at some fancy that had gone before she could remember it. Sometimes her sleep was so operatic that she brought on an asthma attack and woke in a state of mortal terror. Julia could be a very annoying person, Amelia and Sylvia agreed. She had a bewilderingly mercurial personality – punching and kicking one minute, a sham of cooing and kissing the next. When she was smaller Julia had been subject to the most profligate tantrums, and even now a day rarely went by without Julia having a hysterical fit over something or other and flouncing out of a room. It was Olivia who usually tagged after her and tried to console her when no one else cared. Olivia seemed to understand that all Julia wanted was some attention (although she did seem to want an awful lot of it).

Olivia tugged at the sleeve of Julia's nightdress to wake her, a process that always took some time. Amelia, in the next bed, was already awake but kept her eyes closed to savor the last drop of sleep. And besides, if she pretended to be asleep she knew that Olivia would climb into bed with her, hanging on to one of her limbs like a monkey, her sun-browned skin hot and dry against hers, the spongy body of Blue Mouse squashed between them.

Until Olivia was born, Amelia had shared a room with Sylvia, which although it held many drawbacks was definitely preferable to sharing with Julia. Amelia felt stranded, vague and insubstantial, between the acutely defined polar opposites of Sylvia and Julia. It didn't matter how many afterthoughts there were – she sensed she would always be lost somewhere in the middle. Amelia was a more thoughtful, bookish girl than Sylvia. Sylvia preferred excitement to order (which was why, Victor said, she could never be a great mathematician, merely adequate). Sylvia was nuts, of course. She'd told Amelia that God (not to mention Joan of Arc) had spoken to her. In the unlikely event of God speaking to anyone, Sylvia did not seem the obvious choice.

Sylvia loved secrets and even if she didn't have any secrets she made sure that you thought she did. Amelia had no secrets, Amelia knew nothing. When she grew up she planned to know everything and to keep it all a secret.

Would the arrival of the afterthought mean that their mother would juggle them around again in another arbitrary permutation? Who would Olivia move in with? They used to fight over who had the dog in bed with them; now they argued over Olivia's affections. There were five bedrooms in all, but one was always kept as a guest bedroom even though none of them could remember a guest ever staying in the house. Now their mother had begun talking about doing out the attic. Amelia liked the idea of having a room in the attic, away from everyone else. She imagined a spiral staircase and walls painted white, and there would be a white sofa and a white carpet, and gauzy white curtains would hang at the window. When she grew up and married she planned to have a single child, a single perfect child (who would be exactly like Olivia), and live in a white house. When she tried to imagine the husband who would live with her in this white house, all she could conjure up was a blur, a shadow of a man who passed her on stairs and in hallways, and murmured polite greetings.

By the time Olivia had roused them all it was nearly half past seven. They got their own breakfast, except for Olivia, who was hoisted onto a cushion and served cereal and milk by Amelia and fingers of toast by Julia. Olivia was theirs, their very own pet lamb, because their mother was worn out by the afterthought and their father was a great mathematician.

Julia, stuffing herself with food (Rosemary swore that Julia had a Labrador hiding inside her), managed to slice herself with the bread knife but was dissuaded from wailing and waking their parents by Sylvia clamping her hand over her mouth, like a surgical mask. At least one incident a day involving blood was the norm – they were the most accident-prone children in the world, according to their mother, who suffered endless trips to Addenbrooke's with them – Amelia cartwheeling her way to a broken arm, a scalded foot for Sylvia (trying to fill a hot water bottle), a split lip for Julia (jumping off the garage roof), Julia, again, walking through a glass door – watched by Amelia and Sylvia in dumbfounded disbelief (how could she not see it?), and Sylvia's strange fainting episodes, of course – vertical to horizontal with no warning, her skin drained of blood, her lips dry – a rehearsal for death, betrayed only by a slight vibration of the eyelid.