I'm the mother of a 10 year old boy, Conrad. I think it's fair to say I'm a devoted mother. He's my priority and passion, the way so many children are to their mothers. I've been very public about how happy I am with having an only child. It's not for everyone but it was the decision my husband and I made, and I never regretted or questioned that decision. Until I hit 40, when suddenly I became desperate to have another baby.

My change of heart didn't make sense on any level: my husband definitely didn't want any more children, the age gap between my son and any potential sibling would be bigger than ideal and it would've been the wrong time in my career.

The three of us were a happy, mobile, loving family; content and balanced, not stretched. Why would I want things to change? I revisited all the factors that had influenced the decision not to have more than one child and found they all still existed – yet something was different because, quite categorically, I wanted another baby.

I wondered if my body was aware that I was galloping towards the last chance saloon. Was there some chemical telling my brain it was now or never? Possibly, but I've quite a few years before it's technically an impossibility. Besides, although I have friends who tell me that their clocks have chimed like Big Ben every month since they were 30, I've never felt that biological need for a baby.

I've never been the sort of woman who sniffed babies' heads or gazed longingly into prams. Even when I was having one I remained quite calm; more inquisitive than exhilarated.

I was young and naive, my vision of being a parent had largely been formed by Calvin Klein adverts, but between being pregnant and hitting 40, I'd been a mother for nine years. I'd developed a sharp sense of reality. I knew what I'd be letting myself in for – going to the loo alone would be precious "me-time" but I also knew how sensational it is to be a parent, to watch a child grow and thrive, to lose my heart completely.

My husband was fed up. We'd agreed not to have more children; he couldn't imagine why I'd renege on that carefully considered decision. He pointed out the conveniences of living our steady, prosperous, settled life – this just irritated me. I accused him of not understanding me (not surprising, considering I didn't understand myself). I resented the fact that he wouldn't budge. I felt alone, misunderstood, frantic, impulsive. I briefly considered getting pregnant and facing him with a done deal. I wouldn't have been the first woman to have taken this route.

All around me, friends and contemporaries were having late-in-life babies. Two were in my position (mothers of only children who'd sworn that was all they wanted) and some had two, three, even four reasonably grown-up children but had decided to start again. I wasn't alone in this urgent desire to have more babies.

I veered from not being able to spend time with people with babies (I was insane with jealousy) to longing to hold one (and pestering practical strangers to hand theirs over). It was confusing and, frankly, heartbreaking. I'm usually a clear thinker, a planner and self-assured enough to take responsibility for my own decisions and actions, but for the first time in my life I didn't know what I wanted or which way to turn.

I finally started to reach some understanding of my own state of mind when I babysat for a friend's three-month-old daughter. I was entirely competent, the baby was cute and obliging, taking her feeds and napping when expected. However (I'm a bit ashamed to admit this), by the end of the day I was dying of boredom. Suddenly, all desire for another baby began to drain away. Over the next few weeks I began to be startlingly honest with myself. Was it really a baby I wanted, and if so how could a genuine desire bleed away so dramatically? I remembered feeling occasional bouts of boredom when Conrad was tiny but largely I remembered being rapt and enchanted with every gurgle, smile and milestone.

I still firmly believed that babies are amazing, important, wonderful things. But abruptly I understood I didn't want another baby, not really. I wanted to be a mum to baby Conrad again. Specifically, I wanted to be 31 again. I was suffering from a mid-life crisis; a kid-life crisis. Is it possible that middle-aged women yearn for babies not because they want a baby per se, but because they want to be the woman they were when they had their babies; a desirable, vibrant, important young being. It's exactly the same as a guy buying a convertible car or chatting up his secretary. It's about resisting the inevitable. Ageing.

I discussed my theory with friends. "Oh yes," admitted one. "It's just so wonderful being pregnant, everyone is interested in you."

"Yes," added another excitedly, "and you've got a purpose."

"You feel so alive. So far from death," contributed a third.

We sounded like a group of errant, middle-aged husbands justifying our affairs. I remembered being irritated with my husband when he pointed out all the conveniences of living our steady, prosperous, settled lives. His words had made me shudder; I'd wanted to feel youthful. I'd moaned that I felt alone, misunderstood; again vocabulary associated with a mid-life crisis.

So here we are, in the 21st century, armed with equal rights but somehow a female mid-life crisis manifests itself in a desire to procreate, not seduce a toy boy, whereas the male mid-life crisis is associated with illicit sex or an expensive, red convertible. I don't know what the answer is. Because I'm probably having a mid-life crisis, I'm all about questions. But understanding my desire for a baby in this way was helpful; once recognised as slightly selfish, my craving started to subside.

Closing the door on my own fertility hasn't been easy, nor is closing the door on youth, but it was vital that I remembered how to enjoy the child I was already blessed with, in the way that errant husbands skulk back to their wives after a dangerous flirtation.

• Adele Parks' latest novel, About Last Night (Headline , £14.99), is out now