When we brought our first baby home from hospital, I laid him on our bed and cried. He was so small. Loving him made my heart hurt

Ollie and Leah have just had their first baby, Ellen. It's a hot, light October afternoon, and she snoozes in a clean, new carry-seat in an immaculate navy velour Babygro a month too big for her. Tiny blue-white fists drowning in cuffs, head lolling on the support cushion, black tufted hair like stuck-on marabou.

"So," Jonathan and I inquire brightly, "how are you? How's it all going?"

"Oh, we're, um..." They yawn and gaze at Ellen as if she might just come up with a reply. "Fine, actually."

"I managed to have a bath the other day," says Leah, picking cat hairs off her sweatshirt. "Uninterrupted. Listened to half of Women's Hour."

"Does she sleep?"

"She went right through until three last night," says Ollie, triumphantly.

"Three?"

"Three am. That's the first time."

Jonathan gives me a wouldn't-be-back-there-for-anything look. Our house recently became a pushchair-free zone; the last, unfinished bag of nappies went to the charity shop nine months ago. I don't want another baby, but all the same, I feel a quick drag of nostalgic jealousy when I watch Leah bend, gather up the handful of warm limbs, sniff that fierce black hair.

She and Ollie are still there, dazed inhabitants of a milky, hazy planet we left long ago. We're in the world of homework, nits, Pocahontas, bed- time tantrums and cardboard boxes randomly glued together. I wouldn't want to go back, but sometimes I do hanker after the unreality, the soft smallness of that other place.

Chloe, four, ours, marches into the room, looks around, bites her lip, tosses her straight, bobbed hair and stomps out again. She looks suddenly leggy, mature, multi-faceted. "God, I can't believe this one'll ever be that big," says Leah, looking down at hers.

"We're looking forward to our fifties," says Ollie. "The peace and quiet of the menopause."

When we brought our first baby home from the hospital, I laid him on our double bed and cried because he was so small, so terrifying. Loving him made my heart hurt.

When Jacob was three days old, Jonathan sat with him in front of afternoon TV. "Look, son," he said, "Mighty Mouse..." and immediately fell asleep, exhausted by the responsibility, the child cradled in the crook of his old, darned jersey. I went out of the room and when I returned, the shock of my love for them both made me cry again.

"Do you remember all that?" I ask Jonathan when Leah and Ollie have gone.

"All what?"

"Being all soft and shell-shocked like that?"

He grunts, and opens a book on medieval town life. I have a quick vision of the blue-ish furred curve of the top of Ellen's head. "She's a lovely baby."

He laughs. "She's just your off-the-shelf newborn."

"You didn't think that about ours."

He smiles, shrugs. I remind him of falling asleep in front of Mighty Mouse. "I'll have to take your word for it," he says.

That night, we leave our own big, shouting children at Jonathan's mother's and go to the birthday party of Tim, my sister-in-law's husband. The street is crunchy with clean, red and yellow Chelsea leaves, the venue crammed with peeling radiators and faux-shabby people and elderly oil paintings. Seventeen of us assemble by the bar. Tim's expecting just a quiet dinner for four.

"You wouldn't ever give me a surprise party, would you?" Jonathan demands suddenly. "I mean, EVER?"

"Of course not." I'm indignant. "I know you better than that. It would be utterly wasted on you."

"Just checking." He ruffles my hair. Someone is telling OJ jokes. A waiter asks Jonathan not to put his glass on the snooker table. A white-haired man wanders up and down the room in a scarlet sweatshirt with 'Life is a bowl of cherries' stencilled in yellow.

When Tim and Jane come in, the air zings with exclamations, kisses, presents. And Jonathan's ex-wife, whom he married briefly at 23, arrives. I barely know her, but we meet now and then at parties and her mixture of ex-wife glamour and friendly, grown-up charisma always makes me feel like Jane Eyre or the second Mrs de Winter - plain, poor and unentitled. Even now, I struggle with the impulse to run away, to take a bus back to my bedsit in Westbourne Grove (where Jonathan found me) and hunch in front of an electric bar heater, eating packet soup with green bits floating in it.

Now, curried parsnip soup is sipped and brown rolls torn apart at a table awash with art school friends of Tim's youth. The light is velvety. Faces glow; years, babies, worries, tears and paracetamol nights all washed away.

"This is wonderful," says Tim, and as I look at him, something inside me empties. For the second time that day, I swallow back sadness. Father and son asleep in front of Mighty Mouse, fingers entwined. Ellen's bite- sized fists in her huge rolled-up cuffs. Happiness has its own bleak momentum. Some things never come back.

"What is it now?" Jonathan whispers.

"Nothing," I sigh. "Just that I do love Tim so much."

"Yeah, yeah, eat your soup."

All around me, laughing faces. Silver spoons, ear-rings. Eyes squeeze and blur in the candlelight. Through the doorway, there's a painting of the room itself from maybe 70 years ago: everything - pictures, curtains, walls, brown velvet light - perfectly mirrored. The people themselves are long gone, but in the picture, they're frozen - mouths open, arms raised, surprised laughter.

Julie Myerson's novel 'Sleepwalking' is published by Picador at pounds 4.99.