George Romney, "A Mother and Children (Two Studies)." Undated. From the collections of the Yale Center for British Art.

To be born in the 1970s was to be slotted into a sequence—a regular, stepped pattern. My siblings and I, the neighbors, the children with whom we played, the families at our school, we all of us had our place in these arrangements.

Women back then tended to have a baby and, two years later, they had another. And perhaps another, in two years more. That magical pair of years was considered the most apt interval or breathing space between children. It was simply the way things were. Like paper dolls, we, the children, could stretch out a hand and there, right there, a sibling would be. Families were neatly ordered, in gently descending height, one following after the other, like matroyshka dolls.

I had been under the impression that, these days, families and their structures tended to be a bit looser than when I was a child, that women were able to make more choices about when and if they would have more children—or, indeed, any children at all. But when my son, who was born in the early noughties, reached the age of two, I began to find that random strangers would take it upon themselves to inform me that it was time to procreate again.

The language of these unsolicited entreaties was oddly intimate, peculiarly euphemistic. You need to get your skates on, was one. Is there going to be a new bun in the oven? was another.

By the time he reached three or four, these comments—handed out to me in ticket queues, during bus rides, at the post office—had become more insistent, more intrusive, as if my failure to produce a second child provoked them beyond measure. Tick tock, tick tock, people said to me, often with an admonishing, wagging finger. I had a neighbor who left her house with the express purpose of telling me that if I didn’t hurry up my son would be a lonely only. Is there a worse way to describe a child? I used to sprint from car to front door, hauling shopping and son, to avoid her.

The truth was that I was trying to have more children. Trying and trying and trying—but failing. There were peaks of hope, followed by troughs of loss. There were scans that showed minuscule hearts, halted in their purpose, and pale, immobile figures, floating like wraiths in dark space. There were months and then years of baffling nothing. My body seemed to have forgotten how to pull off that particular trick. Every twenty-eight days felt like another loss, another failure. My son gazed wistfully at his classmates’ babies. He wished with every birthday candle for a brother or sister. One day, he asked me if it was possible to play tag on your own.

Infertility is, in all its forms, a most interior, closeted anguish. Nobody wants to discuss the workings of their most private spaces. Secondary infertility—the inability to carry or conceive a second or subsequent child—has its own specific heartaches. You are of course lucky because you have a child, and you know that not everyone can say that. But what can hurt is the assumption that if a woman can have one child she can have another—a misconception which results in the kind of knife-through-the-heart comments like those of my neighbor. Those who have suffered primary infertility have told me that the only way to get by is to avoid everything and anything to do with babies. For the secondary infertility sufferer, this is not an option. You’re confronted on a daily basis at the school gates by pregnant women, people toting slings, older children playing with toddlers, large families squashed into multiple buggies. School drop-off becomes a terrible tableau vivant of all the things you want but cannot have.

The highly desirable, magical two- or three-year age gap had long receded into the distance when we went to see a fertility doctor. He probed and prodded and drew blood and produced graphs and pronounced that there was nothing amiss with either of us. There is, he said, no reason you can’t have another child. I sighed tearfully, my husband cleared his throat. Then why, we asked, isn’t it happening? The doctor couldn’t tell us but he offered us two things. The first was the adjective “unexplained,” to add to our diagnosis: we now had Unexplained Secondary Infertility. The second, an offer of IVF.

My daughter was born the following year. Three years later, after more miscarriages, I had another daughter. So, having thought for years that I would only have one child, I now have three and I still have to pinch myself, daily, to believe my good fortune.

My children, then, come in two sizes: XL and S. My son, who is nine years older than his youngest sister, is unusually tall for his age. He towers over us all. He’s 14 but could pass for much older. Someone asked me recently what he was studying at college. He has to show his ID on trains or in cinemas to be allowed a youth ticket. My middle daughter, by contrast, is petite, light-boned, agile. She is less than half her brother’s height. He can pick her up with one hand—and frequently does.

Having an adult-sized person on hand does, as any mother will tell you, have its uses. If my youngest suddenly decides she absolutely cannot take another step, her brother will hoist her to his shoulders and he, it has to be said, is much more fun a parent in these situations. A rash and reckless ride will ensue, alarming for me but thrilling for her. He will push the stroller through an airport, while I juggle tickets and carry-on bags. At a push, he can be persuaded to keep a life-guarding eye on them if, on holiday, they are in the pool and I need to go and prepare lunch.

His style is less mature and responsible carer, more Cat in the Hat with his two small sidekicks, Thing One and Thing Two: he loves nothing more than whipping them up into a frenzy of reckless abandon. He can enter a room where they are sitting at the table, serenely colouring in, and with just one, carefully chosen trigger word, can transform them into shrieking dervishes, delightedly flinging themselves off furniture. He, naturally, will then exit, grinning widely, leaving me to deal with the consequences.

There are challenges to our setup, as well as joys. Ever tried finding an activity, on a wet weekend, that will entertain both teenagers and small children? Would my son like to visit the baby goats at the farm park? Not much. Would my daughters be happy on a 20-mile bike ride? Resoundingly, no.

Shielding the younger ones from aspects of the teen years can also be tricky. Here is a list of words you cannot utter in front of them, I tell my son. Happy as I am to discuss global politics and terrorism and nuclear threat with you, I say to him, we can’t do it when your sisters are in the car.

Like any parent, I worry about the future. I foresee trouble ahead when my son goes off to college or leaves home. At least I know that his departure will happen; I may dread it, but I’m aware of that impending reality. For his sisters, however, the idea will be inconceivable, incomprehensible. They have never known our house, our lives, without him; they will still be children when it occurs. How will they cope without his anarchic interventions, without his madcap wrestling bouts, without the sound of his trumpet practice, late at night, as they are drifting off to sleep?

This was at the back of my mind when I was writing my novel, This Must Be the Place, which features an unconventional family living in the wilds of Ireland. In one chapter, the six-year-old daughter wanders about the garden, desolate and lost, missing her much older brother, who has gone away; she is unable to articulate this enormous and unforeseen loss.

The word “gap” has two main meanings: a break or hole or absence of something; and an opening, a route forward, as in a “gap between mountains.” I prefer the second interpretation, of course, when using the phrase “age gap,” because I wouldn’t, of course, have my family any other way now. Even if a genie were to appear and offer me a rewind and erase on my years of reproductive problems, I wouldn’t do it. There isn’t an atom of my children’s beings that I would change. Our age gap, for me, is a testament to what we all went through; it’s evidence of love, as well as loss. It’s woven into the fabric of who we are. It’s a visible reminder of how fortunate I’ve been.

People do look at us slightly askance, on occasion. There is a particular expression which appears on the faces of shopkeepers, bus drivers, delivery men. I’ll see it on the faces of waiters who step up to our table to take our order, on the features of the woman in the shoe shop before she stoops to measure my kids’ feet. It’s mostly benign: part surprise, part confusion, part curiosity. The gaze of these people will pass from me, to my children, and back again. Eyebrows are often raised; mouths are hesitantly smiling.

Most of the time, these people don’t vocalize their thoughts. Occasionally, they can’t hold it in. “Are they all yours?” I’ve been asked. An uninhibited taxi-driver asked if they were from different marriages, which I felt was a step too far and declined, politely but firmly, to answer. A woman at a gym class once surveyed them and said, with an insensitive cackle, “It’s as if there’s a child or two missing!”

There is solace to be found in the knowledge that having children with big age differences is a secret club. I was in a supermarket recently and I passed a woman with an older boy and twin babies in the trolley. I saw her clocking my children and their ages; she saw me doing the same. We looked at each other for a moment. I smiled at her, she smiled back and then we walked on, trailing children in our wakes.

Maggie O'Farrell

Maggie O’Farrell is the author of seven novels. Her memoir, I Am, I Am, I Am: Seventeen Brushes with Death, will be published by Knopf in February.

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