“When did you know that you wanted to be a solo mum?” reporters and friends and strangers often ask me. They wait for a story of a life-altering revelation that crystallised my decision and rerouted my path. That’s because the way we tell – and write – stories, the heroine is usually catapulted out of her life by an event. Lightning strikes. Strangers collide. A ringing phone brings the news. And everything changes.

I’ve crafted various answers to those asking when I chose to become a mother on my own; I’m a writer after all, I can spin a winning anecdote. But none of them are true.

I could say that my decision was cemented on the last pre-baby date I went on – around the time dating had completely lost its lustre, when I was filled with seething frustration at the Peter Pans I met who were never going to be ready to make a family. One night, sitting across the couch from a handsome actor who’d spent the night moping through our fifth date, he confessed he wanted to be polyamorous.

Given how uninterested and noncommittal he’d been, we were barely anything amorous. So when he said the word polyamory, my jaw dropped and I let out a Munch-like scream that was powered by the force of a thousand angry witches, propelling the actor out my front door. OK, I didn’t actually scream but if you had peered inside the vortex of my mind, you would have seen a small figurine of myself screaming. Instead, I smiled, I played nice, I went along with the rest of the date.

You could say that I knew I would eventually be a solo mother years before, back when I was living in New York. One lunch break, I took the elevator down from my daylight-starved office cubicle and emerged a few blocks from Times Square. I plugged in my headphones as I beelined past the tourists and jaywalkers, the rubbish and the blaring advertisements, and over the gum speckled pavement. As I walked, I listened to an interview with Alyssa Shelasky, a sex and dating columnist for New York magazine. She’d been living in Italy, in a long-term relationship, until suddenly she wasn’t. She left Italy the day after their breakup and flew back to New York. Shelasky was approaching her late 30s and badly wanted to have a baby, but instead of boarding the dating rollercoaster again, she’d decided to get pregnant on her own, using donor sperm. Hearing this, I felt a lightness, my heart fluttered. Shelasky had become a “solo mother by choice”. I silently mouthed this new term that I’d never known about, learning it by heart.

I often tell people that I knew I’d try for solo motherhood after the final breakup – when I realised through sobs that I wasn’t mourning the man I was losing, but the future family we’d never make.

All of these stories can be presented as pivotal moments but they’re a lie because like most large decisions in life, there was nothing sudden about it. I inched my way towards choosing solo motherhood. I moved in increments so small that if you tried to dramatise it as a scene in a play, you’d see me doing a Butoh-theatre-style-slo-mo walk as I made my way from one side of a room to the other.

There were hundreds of cups of tea and minutes spent late at night scrolling; there were solo mums’ support meetings and texts with friends and silent walks along dark streets lined with silent terrace houses where I imagined real families lived. All of these eventually amounted to a decision. It took years as I wrestled with the choice, and grieved one imagined life and chose another. So that finally, one day, I was lying in a doctor’s office, legs open as she inserted a catheter of sperm into my uterus.

And now all of those increments have become a small person, who is three.

It’s an ordinary night like any other when he clambers into my bed at around 2am. He crawls up to me and lies with his head pressed in between my shoulder blades, his back warm against my spine. During the night his body rotates to horizontal – his feet landing in my face. Where once I lay in bed and he was inside my belly kicking me, now he has emerged and kicks at me during sleep from outside the confines of my body. Mostly I don’t mind because he is my beloved, my son, and I am still intoxicated by the soft, small not-quite-a-baby but not yet a boy-child of him. By the tender, cherub-curved fat of his cheeks, his unlined brow, as he exhales loud snuffly breaths through his mouth.

His kicks have become so pedestrian, I can sleep through them. So ordinary, they’re hardly momentous. But they make up the inches of a life, of a story, that I wouldn’t want any other way.

• Inconceivable: Heartbreak, Bad Dates and Finding Solo Motherhood by Alexandra Collier is out now from Hachette