This is Kiera. She's three years old. We were at the park, when I took this photograph. Up and down the slide, climbing on the monkey bars, begging me to push her on the swing, again and again.

"No," I said. "No. Mummy's too tired, too big. The baby in Mummy's tummy makes mummy so, so tired. No. No swings."

And though her face, for the briefest of moments, showed disappointment, that was fleeting. It passed, and again she was off on her way, chasing after a butterfly. A red-yellow-black burst of summer.

These are the things that I remember.  The sunshine, the laughter, the way that I said "no". But how was I to know what would come next? Life changes in the flash of an instant, in the flutter of a butterfly's wings.

This is Sophie.  On the fifteenth of September it will be her fourth birthday. I see her now, sitting on the floor, in front of the television, drawing pictures of flowers, faces, spiraling circles again and again.

"Mummy, draw with me?" she asks.

And I am busy, I am working. I have invoices to send, emails to answer, and that important deadline ticking ever close. But I stop.

"Yes," I say. "Yes, Sophie. Of course."

And we sit on the rug, while Peppa Pig jumps up and down in muddy puddles behind us.

"Draw me a dinosaur. Draw me a dog. Draw me the World."

And I do. I do all of this. The clock ticks, the computer switches itself to the screensaver. The family photographs play out on the monitor in a looping cycle. The emails are ignored and the deadline is put off. Because what matters is Sophie.

On her piece of paper, Sophie has drawn us. Mummy, Daddy, Sophie and Kiera. Kiera has wings, like a butterfly.

Like an angel.

And that picture, well, you know, I want to take hold of Sophie and hug her and kiss her and never let go of her. Because life changes in the blink of an eye.  It can go from a girl chasing a butterfly to a girl disappearing. From laughter to tears to sobbing that you feel will never ever stop. To pain that you know will never end.

"Kiera's my angel, Mummy."

"Your big sister," I say, and it hits me.  Sophie is older now than the girl in the photograph. The last picture that I ever took of Kiera. Sophie is the big girl now.

As the years have gone by, I have gauged Sophie's progress against what I knew from Kiera. Sophie spoke sooner, Kiera walked sooner. But now - what? What did I know about being a mother of a four year old?

Only to never, never let her out of my sight.

On the fireplace are photographs. Mummy, Daddy and Kiera together. Smiling, happy, together. Then Sophie, Mummy and Daddy, but never Sophie and Kiera. Never. My two children will never meet.

"Read to me, Mummy," Sophie says, but I am looking at her drawing and I have to bury my face into her soft hair to hide my tears.