The moment it became obvious I was in serious trouble happened in the waiting room of our local GP surgery.

There he was, just one Formica chair away, swaddled in the softest baby blue blanket, snuffling into the shoulder of his mother as she absent-mindedly patted his tiny rotund bottom.

The cuteness! Like a hot flush I felt it coming — driven by an instinct deep-rooted inside me — and as his mother was flicking through an old women’s weekly, I reached out and longingly stroked his downy little head.

‘Mum! Don’t be weird,’ my 18-year-old daughter Flo hissed from the seat next to me. Chastised, I quickly snatched my hand back, but not without the most dangerous of thoughts running through my head. Another baby. I could still, couldn’t I?

Aren't they enough? Shona with, from left, Dolly, Monty, Annie and Flo

Oh, the irony. The fact that I — aged 45 — was sitting in that surgery waiting for Flo’s contraceptive prescription to be printed did not, for a single painful second, escape me.

On one side, my sexually active, eldest daughter on the brink of everything I so desperately wanted back.

Unashamedly fertile, and why shouldn’t she be? Dear God, I only had to hang on a few more years and she would most likely make me a grandmother.

And on the other side, that baby. Without trying to sound like a complete loon, I wanted to grab him, stick my nose in the folds of his warm neck and deeply inhale. I wanted, with a yearning I haven’t felt for years, for him to belong to me.

Is this some form of middle-aged, pre-menopausal madness? It certainly feels like the cruellest joke in the world. I wasn’t even this broody in my 20s when I had an entire nightclub of hedonistic eggs in my ovaries all clambering to collide with Mr Right.

But it seems I’m not alone. This month, mother-of-four Nicole Kidman — who turns 50 in June — bravely admitted she would ‘never say never’ to having another baby.

When I heard this I have to admit to experiencing a kind of revelation. There I’ve been, for months now, hiding my shameful secret. It feels almost gluttonous to finally confess.

After all, having given birth and raised four healthy babies, haven’t I had my fair share? I have no right to be sniffing other people’s newborns and craving a fifth baby at this alarmingly 11th hour.

But perhaps it’s not so uncommon, this defiant urge to stick two fingers up at Madam Menopause at the very moment she beckons from round the next bend?

I have no right to be sniffing other people’s newborns and craving a fifth baby at this alarmingly 11th hour

After all, Halle Berry was almost 47 when she had her second child. ‘This has been the biggest surprise of my life, to tell you the truth,’ she admitted at the time. ‘I thought I was kind of past the point where this could be a reality for me. So it’s been a big surprise and the most wonderful.’

She’s not in such crazy company either. Susan Sarandon gave birth to her second child at the age of 47, and David Bowie’s wife, Iman, having had one child from a previous marriage, gave birth to her daughter with the rock star when she was 45.

Indeed, it was the news that my oldest and best friend from school was herself pregnant, with no help from IVF, at the age of 45 that made me seriously consider the possibility of squeezing another one in.

I have to admit that when she told me her news I felt more than an appropriate tinge of envy. I might even have cried, just a little bit.

Which is so unfair on her. She tried for years to have her first daughter, my god-daughter, at the age of 40, and this pregnancy five years later is nothing short of a marvellous miracle.

But as she blossoms and nests preparing herself for, let’s face it, what has to be one of the most exciting experiences a woman can go through, I am dealing with the other, less rosy, end of motherhood. The bit where there is no soft bundle to nurture and hold, and nobody to make me feel that I am crucial to their life on earth. On the contrary, the only time nowadays I am needed is to find clean socks and provide a chauffeuse service.

I often wonder if this is the root cause of my mid-life baby hunger. Do I want another baby because I feel so washed up and non-essential? Because, honestly, parenting teenagers sucks.

This month, mother-of-four Nicole Kidman — who turns 50 in June — bravely admitted she would ‘never say never’ to having another baby

David Bowie’s wife, Iman, having had one child from a previous marriage, gave birth to her daughter with the rock star when she was 45

Halle Berry was almost 47 when she had her second child and Susan Sarandon gave birth to her second child at the age of 47

It is a constant, daily, reminder that life is hurtling by and that I will soon be sitting in a child-free house with my husband Keith, drumming my fingers and wondering where all the years went.

And isn’t that the awful juxtaposition of motherhood? I remember when Flo was four, Annie, two and Monty just a tiny newborn. God, I was exhausted. The nights often felt interminable, the days never-ending.

But fast-forward to today and I would happily sell a kidney for the chance to turn the clock back, if only for an hour, to experience that moment in the maternal spotlight once again.

So is that all I’m experiencing? Nostalgia, clashing dangerously with self-pity — all dressed with a strong dose of panic over the advancing years and my all-too-apparent mortality?

Apparently not, says renowned hormone expert, Marion Gluck of the Marion Gluck Clinic. After more than 25 years helping women with the transition to menopause, she says it’s entirely understandable — and surprisingly common — that I’ve suddenly been transformed into this baby-sniffing, weepy lunatic. In fact, the phenomenon is firmly rooted in human biology.

‘Women have lots of oestrogen surges pre-menopause, many more than in their 30s and early 40s,’ she says. ‘You might not be ovulating so much, but it is still very common to experience hormonal imbalances. There is only a limited time left and this is the “last Hurrah”.

‘As women we are dictated to by our hormones — they drive everything we do. And we are also nurturers. So it is entirely understandable to begin to mourn an event that may never happen again. Indeed, I have heard many hundreds of women bemoan the very same thing.’

I could explain all this to Keith but, quite honestly, I feel it would be pushing my luck. After all, we’ve been here once before.

A year after Monty, who’s 14, started school in 2008 I had a similar maternal meltdown. With all three of my children now at school I remember feeling torn. Life was, undoubtedly, much easier. We’d survived those rollercoaster early years, a little battle weary but, nonetheless, intact.

Looking back, I’m fairly certain Keith felt we were on the home straight back to our old pre-child existence of lie-ins and romantic spontaneity.

I waited until we were on holiday in France that summer to drop the bombshell. ‘I want another baby,’ I announced out of the blue just as he was heading out of our Gite for a run.

‘Hold that thought . . .’ he replied, before turning slightly puce and scarpering off down the road at breakneck speed.

I did wonder, for the next hour, if I would ever see him again. But one thing Keith isn’t is a quitter. He returned home in an oddly contemplative mood.

Shona with her au pair Fabien and Florence, Annie, Monty and Dolly

‘I’ve just jogged past an elderly couple holding hands on a bench and looking out to sea,’ he said, planting a sweaty kiss on my forehead. ‘I don’t want you to regret anything when you’re old and wrinkly. So if you want another baby then we should go ahead and do it.’

This is quite possibly the most romantic thing my husband has ever said to me. And so we did.

Dolly was born in August 2009, coinciding perfectly with our tenth wedding anniversary. Among the raised eyebrows of our friends we felt she was the icing on our cake, the cocktail cherry planted firmly on the rim of our overflowing cup.

Except now, seven years on, it appears I want another cocktail cherry and maybe even one of those little umbrellas with a jaunty fringe. It’s OK, I’m thinking it too. I’ve never known when to stop.

Even when the risks are spelled out to me: that, if I did fall pregnant, I’d be three times as likely to experience diabetes and high blood pressure. I’d also be three times as likely to miscarry, and the odds of the baby having Down’s syndrome shrink to one in 100 after 40. How could I even contemplate putting myself through that?

And yet, here I am, browsing baby shops under the pretence of looking for a present for my friend when really I am allowing my mind to go places it has no right trespassing into.

Could I have a ‘little accident’? I seriously doubt it. With three teenagers wandering the house until all hours, we’re hardly swinging from the proverbial chandeliers. And the days of dangerous ‘Should we? Dare we?’ Russian Roulette sex are a distant, delicious memory.

Shona and her brood:'Is this some form of middle-aged, pre-menopausal madness? It certainly feels like the cruellest joke in the world. I wasn’t even this broody in my 20s'

And then there’s Keith, 49. While he hasn’t exactly said ‘no’, he hasn’t said ‘yes’ either. He probably thought I was joking when I casually brought the subject up a while back, and I recall him spluttering something about not wanting to still be dealing with teenagers in his 70s.

But still. After 18 years of marriage, surely any new addition to the family would, eventually, be welcomed as an exciting, unexpected development?

Deep down, I know this is the stuff of fantasy. I am experiencing a kind of insanity that is making me behave — and consider — things no person in their right mind would.

I’m a middle-aged mother juggling the incessant demands of three hormonal teenagers with the plethora of problems that brings: GCSEs, online dangers and boundary-pushing — day in, day out.

And then there’s Dolly, who needs another level of mothering: story-telling, teeth brushing and timetables. Anyone can see this is rocky terrain to be crossed with a mewling infant on my back.

Perhaps, as Marion Gluck advises, it’s simply a case of accepting what will never be. My body might be screaming: ‘Quick, while stocks last!’ But when was panic-buying ever a good idea?

‘It’s a grief process,’ she explains. ‘You need to embrace and acknowledge what is happening to your body in order to fully come to terms with the fact that those days are firmly behind you.’

If only it were so easy . . .