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Last Saturday, around 20 teenagers gathered in the basement of our Surrey home for a party, the result of some fairly epic pleading from my 15-year-old son, Will.

My husband Phil and I had reluctantly capitulated, but the rules were clear: no drinking, and the music couldn't be too loud either — something I policed by popping down every half hour.

It didn't make for the most relaxing evening, especially as upstairs my two-year-old daughter, Isabella, was refusing to go to bed and indulging in the sort of epic tantrum only a toddler can pull off.

Pacing between her fairy light-strewn bedroom and the teen-strewn basement, I had to flick from consoling cuddles to firm-but-fair matriarch.

Because here I am, aged 50 — complete with my own particular brand of raging perimenopausal hormones — mum to a toddler and a teenager, straddling worlds between an adorable but feisty girl who loves cuddly toys and princess dresses and an equally adorable but strapping 6ft 2in young man whose main preoccupation is the latest FIFA pick and his Xbox. I'm trying to potty train one while guiding the other through his GCSEs.

When not changing my daughter's nappies, I'm picking up my son's discarded clothes from the bathroom floor and plucking up courage to enter his messy den, with its mass of trainers, music posters and electronic equipment.

Here I am, aged 50 — complete with my own particular brand of raging perimenopausal hormones — mum to a toddler and a teenager, straddling worlds between an adorable but feisty girl who loves cuddly toys and princess dresses and an equally adorable but strapping 6ft 2in young man whose main preoccupation is the latest FIFA pick and his Xbox. I'm trying to potty train one while guiding the other through his GCSEs.

For some of the day I'm reading Wonky Donkey and watching Frozen, other times I'm trying to navigate complex and difficult conversations about spots, sex and drugs with an embarrassed teen.

It's a wonderful, sometimes surreal world of extremes — one which I never planned but happened courtesy of my ten-year quest to give Will a sibling.

Phil and I married when I was 29 and he was 34, but I was 32 when we started trying for a baby. It didn't happen overnight as it transpired I had endometriosis which needed to be treated before I could get pregnant. But in October 2005, Will arrived, a gorgeous bundle who captured my heart instantly.

I loved being a mum and always planned on two children, but for the first three years of Will's life I couldn't really contemplate having another. I had a demanding job in event organisation which, along with a house move and renovation, meant life was busy enough. By the time Will turned three, however, I was 38 and the clock was ticking.

But the years that followed resulted in just one natural conception shortly after I turned 40, which ended in a heartbreaking miscarriage, then rounds of IVF — cycles of hope and despair. As my 43rd birthday loomed, we took the decision to draw the line. I felt I was too old and couldn't take more crushing disappointment.

Yet the yearning for another child never left me and when, as part of my job, I oversaw seminars at The Fertility Show and learned about egg donation, I couldn't stop thinking about it.

Although 47 by then, I told Phil I wanted to have one last go with a donor egg. It would mean any baby wasn't biologically mine, but that didn't bother me. I desperately wanted to be a mum again.

Phil wasn't convinced at first but finally, he agreed, concluding you tend to regret not the things you do but the things you don't.

I was 47, mum to an adolescent boy — and pregnant! While Phil and I were part-thrilled, part-terrified, Will was little short of mortified. He had been really keen for a sibling, but now 13, full of adolescent hormones and teenage angst, he tried to process the horror of having a middle-aged pregnant mum. 'This is going to ruin everything, Mum,' he shouted more than once.

We chose a clinic in Spain with an impressive success rate and I was implanted with an embryo from our chosen donor's egg, fertilised with Phil's sperm. Amazingly, the procedure worked first time.

I was 47, mum to an adolescent boy — and pregnant! While Phil and I were part-thrilled, part-terrified, Will was little short of mortified. He had been really keen for a sibling, but now 13, full of adolescent hormones and teenage angst, he tried to process the horror of having a middle-aged pregnant mum. 'This is going to ruin everything, Mum,' he shouted more than once.

Will's discomfort was matched only by the shock on the faces of my contemporaries when I told them the news. Many of my friends were embarking on HRT, while here I was taking folic acid.

We hadn't told anyone what we were doing, as we knew people would have an opinion about my age, but after the initial shock most were thrilled for me, although a couple were clearly baffled. 'How on earth can you start all over again?' more than one asked.

If any of them thought I was selfish, they never said so. Although I'd wondered myself, knowing I'd be 70 when my daughter turns 21.

Yet today's 50-year-olds are different creatures. I'm strong, healthy and fit. I may not have 50 years left to see my daughter grow to middle age, but hope to have at least 30. In a world where none of us knows what lies round the corner, it seemed a fair roll of the dice. But as my pregnancy bump grew, I felt self-conscious about being such a geriatric mum and, unlike with Will, no one ever asked me if I was expecting. My age made them too nervous.

In the small hours, meanwhile, I worried that I might not bond with this baby whose DNA came from an unknown woman.

Those nerves disappeared in a heartbeat after the birth of Isabella by caesarean section in April 2019. The moment she was in my arms, it was an instant love affair.

Despite his misgivings, Will also fell in love with his new baby sister from the start. Watching the tender look on his face as he cradled her in his arms for the first time is a moment I'll never forget.

Yet there's no question the arrival of his cheeky, livewire little sister turned our lives upside down. By the time you've got a teenager, you've forgotten how to look after a baby and a toddler, and it took Phil and I time to adjust to the long-forgotten timetable of night feeds, sterilising milk bottles and nap schedules.

Two and a half years on, it can still feel surreal, flicking between looking after a small being completely reliant on you, to supervising another who wants independence, between a little person in bed at 7pm and another endlessly trying to find a way round his 10pm weekend curfew.

Sometimes I smile when I look at my supermarket shop, swimming nappies nestled alongside the low-alcohol beers I sometimes let Will take to a party. Throw in that 'at 50' I'm firmly in the perimenopause zone and it's little wonder that, against a backdrop of teen hormones and toddler tantrums, Phil sometimes jokes he needs to don a suit of armour before coming downstairs.

There are times when the juggle is challenging, but I am a different parent this time around — more patient and relaxed.

With Will, I read every pregnancy and baby book going, anguishing over every supposed milestone, convinced something was wrong because I'd read that, by two, Will should have a vocabulary of 20 words, when he only had ten.

If anything, Isabella's speech is developing even more slowly, but experience tells me she'll be chatting away in the end.

Similarly, the frantic rota of music classes and baby activities I packed Will's days with have been replaced by a gentler timetable of pottering, while the expensive wardrobe of clothes for my first-born has made way for fabulous charity shop finds and supermarket buys.

I like to think the more laid-back me feeds into how I am with Will too, though undeniably the arrival of 'Cat-Cat', as he calls Isabella, has impacted on his world in ways we couldn't have predicted.

I remember the dismay on his face the first time I couldn't come to watch him play rugby because of Isabella's nap times. When we visit friends for lunch, he can get exasperated when I call upon him to leave the teen gang to look after a two-year-old. It's hard not to feel a flutter of guilt, though I tell myself there would be moments like these whatever the age gap.

And their relationship is heart-warming. Isabella adores Will, and from the moment she wakes, wants to knock on his door and jump on his bed. He, in turn, is very sweet with her, playing hide and seek with her, or helping with her balance bike technique.

Yes, I feel a pang that Isabella won't have the same experience of hands-on grandparenting that Will enjoyed: Phil's dad is now 85, while mine is 80 and my mum in her 70s. Yet this small supercharged human has given them a new lease of life. During lockdown, their FaceTime with her was a genuine joy to behold.

Of course, I wonder what it would have been like raising two children nearer in age. But I also remember how many of my peers seemed to be waiting to emerge from the chaos and exhaustion of those early years into the sunlit uplands of their children being six or seven. For all the challenges of my toddler-and-teen parenthood, I've never felt that way.

The day after that stressful teenage party, Phil and I travelled to London for a day out with Will, his girlfriend and Isabella. We went shopping, ate ice creams, and I watched as my son and his girlfriend walked ahead, each holding Isabella's hand. That scene was everything I once yearned for and, while it didn't look like I once envisaged, it's every bit as precious.

As told to Kathryn Knight. Laura Biggs runs The Fertility Show, this year it is online from September 24-27 (thefertilitysummit.com) and Let's All Talk Menopause webinars offering expert help and support every other Tuesday from October 12.