Past 10pm and we can finally "relax" after the marathon that begins with settling our 16-month-old to sleep, followed by a more protracted battle to get our eight- and 10-year-olds upstairs. If we drag ourselves to bed in the next half hour, the hope is the insistent cries of a toddler waking us at 6am will be more bearable.

Slumped on the sofa, not even the twin joys of sauvignon and Nordic noir can keep me up these days. "Maybe we felt this tired first time around," I say, half-dead. Husband shakes his head rigorously. "We’re in our mid-40s - with a baby - that’s why it feels like this. Simple." I can’t help noticing the recent spray of silver hairs around his temples; I dye my own roots monthly. Since her birth, I keep obsessively doing the sums - by the time she’s at primary school, I’ll be almost half a century old. I’ll probably be drawing a pension when she enrols at university, and so it goes on. Each day I watch how my ageing hands contrast so acutely with that plump sheen of 16-month-old skin.

This, then, is a taste of middle-age parenthood when you surf the fertility deadline maybe a little too far, squeezing in baby number three at 45. I look at the two of us, deep into middle-age now, and think about people we know with children starting secondary school, or with teenagers, or childless, but none we know have embarked on this particular sleep-deprived journey.

According to recent figures showing that the number of women giving birth in their 40s has trebled in the last 20 years, I should be tripping over other mothers like myself - the wrong side of 45, losing the battle with grey hairs and laughter lines, but still with a babe in arms. I’m not sure where they are, but I haven’t spotted many at my school gates. Maybe they’re so exhausted, they don’t get out much.

One pregnant fortysomething mother of two did stop me in the park recently to say what an inspiration I’d been from afar; the fact that I wasn’t wearing elasticated trousers one year on was most reassuring, she said.

It confirmed my suspicion that to have a planned mid-life baby, especially with older children, is still an exception, not a lifestyle trend as many magazines would have you believe. Regardless of that trebling in fortysomething pregnancies, I feel I’ve struck out on a rather unconventional path; when life was getting easy and predictable, I chose the swerve ball, parting company with the experience of most of my parenting peers. I’m still a bit of a novelty at the school gates. ‘‘My husband wondered if it was an accident?’’ one mother asked me when I was around six months pregnant. Now I get a lot of, ‘‘I really admire you. How do you do it? I struggle with two older ones, and you’ve got a baby to juggle.’’ Read, ‘‘I wouldn’t swap with you for the world. You must’ve been completely insane.’’ More than a few days a week, especially Sunday evenings, I would probably agree.

Sometimes I look back and wonder what strange, irrational urge took grip. It began as a discreet niggle around three years ago but soon escalated and I found myself, aged 43, desperate to have another baby. Not just any baby, you understand, but a third child. There was something about three, I reasoned - if such a thing was possible in my less-than-logical state. Whereas two were too neat and ordered, three was chaotic, abundant, life-affirming.

Imagine the extra dynamics and relationships that could evolve with a third. Many women in their 40s face a similar battle between mind and body, at least that’s what it felt like for me, a primal reckoning with my reproductive self. Exhaustion is temporary but regret could last for ever, I kept thinking.

Others wisely decide to ignore this deep internal conflict, buy a dog, write a novel, let it pass. As one mother-of-two said when I told her my news: ‘‘I’ll never have another, however loud my ovaries scream at me.’’ Maybe I was weaker - my ovaries won out. By this time, we were both bored by the endless discussion of should we, shouldn’t we. As someone told me, the best reason to have three children is so you can stop talking about having three children. We stopped talking about it and, around one month later, I became pregnant.

So much for hoping that, third time, I may be lucky and avoid morning sickness. This pregnancy was worse than the other two combined; I was sick continually until around week 34. On the plus side, people were more sympathetic earlier on - my muscle-stretched middle-age body yielded to pregnancy within days of the creeping blue line of my pregnancy test, so much so that by week 10 commuters were happily offering me their seat on subway trains.

Far from being treated as a medical concern, my GP was reassuringly blasé. I had low blood pressure and a history of normal natural births - much more significant than my age, I was told. It was a similar story at my maternity hospital where my midwife shrugged, ‘‘So what - we had a 49-year-old in last week who gave birth to her first.’’

As it happened, my birth was nowhere near as easy - or as natural - as the other two, which was partly age related. Because of my age I was induced a week early, which - due to complications - led to an emergency caesarean 12 hours later. It was a pretty hideous birth - Amelia wasn’t breathing when she was born, although she recovered soon after - and a brutal reminder that going for it at this late stage in life had been a real risk. It took me many weeks to accept that Amelia was healthy and fine, and to stop feeling guilty that my selfish yearning could have had a potentially disastrous effect on the family.

Compared with the other two, her existence seems more miraculous as a result, but more of a marathon too. One fantasy of fortysomething motherhood was that somehow I’d be more mature and calm about handling a tiny baby. No chance. If there’s one thing I’ve learned about parenting, it’s that experience seems to count for very little. The moment she slept through I was terrified, just like with the other two. "Enjoy the lie-in while you can," mutters my husband and turns back to sleep. "How can I? She may have stopped breathing."

I’m no more relaxed when Amelia is pink and floppy with her first soaring temperature. I check obsessively for rashes, glass tumbler at the ready. We still find the whole experience bemusing. Every so often, last thing at night when the house finally falls silent, my husband will say, "I can’t believe there’s a baby downstairs, can you?"

In the early weeks, it felt unreal seeing my husband weaving the canvas straps of a baby carrier around his waist once more; as did the bundles of tiny nappies, the smell of baby talc and nappy cream. Are we really here again?

Yet, 16 months on, watching the dynamics develop between the three of them is as satisfying as I suspected. Evie was frosty to begin with, and in the early weeks would roll her eyes and mutter, "Does she come with an off button?" whenever she began to cry. Now she is protective and playful, buying her clothes and selecting matching outfits.

We’ve seen another side to Louis too, more nurturing and indulgent. He’s the first one into her room to comfort her if she’s unsettled, chiding me if I dare to leave her to cry. I ask my husband, after his early-morning shift with Amelia, how age has changed his experience of parenthood. Yes, he finds it more relentless, he says, but is more appreciative of the bits he forgot first time around; the moments of intimacy rocking a small baby to sleep, their physicality, the way their arms cling to the back of your neck; all that disappears with bolshy eight- and 10-year-olds.

It is lovely, too, seeing the first signs of a gang emerging - the three of them slouched on the sofa, Amelia wedged between the older two, glued to the iPad. It also turns out that what I was most apprehensive about - the yawning age gap - has been the best thing of all. Louis and Evie will remember much more about their sister’s early life than they do about each other’s, and I’ve appreciated the mutually exclusive time during school hours; it’s rather like having an only child. This mid-life baby will be lucky enough to have two older siblings she can fight and compete with but also turn to for advice, love and support - with her exhausted ageing parents, good thing too.

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