There are not many moments like this. Moments that split open your world, slicing a deep crevasse across your life, so that everything before the moment belongs to a foreign, unvisitable world where a language is spoken that you no longer speak, and the words and customs of the new world are suddenly all that is comprehensible to you. It might happen after a big death, or the birth of your first child. Or it might be upon hearing a particular string of words uttered at that right, rare moment when your heart is raw and open.

That is what happened to me. I knew my child was different from the other children. I knew that most three-year-old boys did not spend long afternoons playing with plastic fairy figurines. I knew they didn’t beg their moms for ballet classes and princess dresses and everything that sparkled and glittered. I knew this was going to be more complicated, raising a boy who did not act like one.

The other moms assured me it would pass. At pre-school pick-up, they would enthusiastically compliment the surprisingly pink shoes worn by my little boy. “What a fun colour!” a mom would say. And as my son smiled shyly and looked down to admire his beautiful feet, the mother of an older boy would tell me of the time when her own son had likewise mistaken the world of girls for his own. “My son loved pink in pre-school, too!” she might say. “He used to dress up in his big sister’s clothes!” She would laugh at her sweet story, an example of the kind of charming error small children often make, like thinking you could draw your own money, or that your parents were old enough to remember the dinosaurs.

But I wondered.

I wondered if her son had ever drawn a self-portrait with puff-sleeved gowns and Rapunzel-length hair. Or recoiled at the sound of his own name, declaring it ugly and pleading to instead be called something pretty, like Rainbow. I wondered if her son had ripped off his clothes every day after school, to replace them with the floral-print party dress coaxed out of his grandma on a trip to the thrift store, and if he had then twirled around the living room in a graceful trance, singing a tuneless song about fairies.

I wondered if this mother had dithered and delayed in response to his ever-pinker requests, hoping this unusual passion would subside with time. If she had lain awake at night wondering where she had gone wrong, asking herself how she had so utterly failed to steer her precious boy in a safer direction, and whether there was any chance left of helping him change course now.

And I wondered how she felt when it dawned on her that all of the characters in her son’s favourite books, and the only children he requested for playdates, and every single one of his beloved stuffed animals were girls. Like me, she had probably never heard of a boy like that – a boy who didn’t seem to want to be one. When the world split wide open, it was a November evening. We had just walked in the front door and were shedding the day’s damp coats and bags. Outside, the sky was preparing for an early bedtime, transforming the cloud ceiling from old-pillow grey to the colour of wet ash.

I reached out to flip on the lights and felt my child slip his hand into mine.

“Mama,” he said, “something went wrong in your tummy.”

I heard my purse hit the floor. “It did?”

“Yes,” he said. “And it made me come out as a boy instead of a girl.”

The tips of his fingers dug into my palm, and I looked down at the three-year-old face tilted up at mine. The perfect brow was creased down the middle. His pale blue eyes, like circles cut from a summer sky, were flooding with tears, but did not blink. His little body, usually in constant motion, was unnaturally rigid and tall, a tiny soldier frozen at attention.

“Breathe,” I said to both of us. “Take a deep breath.”

He ignored me. “Put me back, Mama,” he rasped, expelling all that was left in his little lungs. “Put me back, so I can come out again as a girl.” He gasped for air and his body curled up into sobs. I sank to my knees and reached for him, but he pushed me away and pointed with his whole arm at my stomach. “Please, Mama!” my child howled. “Put me back!”

The evening’s last grey light was gone, the living room windows had turned black, and the door to the kitchen was now a bright rectangle filling the room with long shadows, including ours, which climbed the wall to the same height, and which were both trembling.

For months, I had been saying no to requests well within my power to grant: sundresses and ballet slippers and Barbie dolls. But now, my child was asking for something I couldn’t possibly deliver. Now, the only reasonable answer to give, unequivocally, was no. And I could not.

I could say no to all those pretty objects, but not to this. Because whatever this was, I knew that it was real, and it was everything. It was the thing he had been trying to tell me, and that I had been trying not to see, for months, despite ten thousand sparkly hints. It was impossible, but somehow it was also true.

I bundled him into my lap and heard myself promise my child something I had no confidence that I could ever deliver. “It’s OK,” I said. “You can be a girl.”

I held his damp head pressed against my heart and rocked him back and forth. He sniffed and shook in my arms while I continued to promise the impossible. “Yes, my darling. Yes.”

We were a single swaying shadow on the wall now, and we rocked and rocked, until his tears were dry and one of us felt safe again.

How To Be A Girl, Icon Books, £12.99stg, is out now.