When I quietly celebrated the news from my 20-week pregnancy scan that I was carrying a baby girl, I never dreamt it would spark a global trend. It was 2008 and my husband and I had got married, bought a house and been trying for a while to start a family. My first two pregnancies had ended in miscarriage, so reaching the 20-week point with my third felt like a major milestone for us. It was one that we wanted to mark.

So we held a small gathering at our home in Chicago, just for family and one or two friends. I baked two cakes: one with blue filling and one with pink, then had my sister-in-law look into a sealed envelope containing the ultrasound image and bring out the appropriate cake.

We cut into it and the whole family was able to see the pink icing at the same time we did. It was a magical moment, but we didn't think much about it after that.

Later, when I wrote on my blog that I'd thrown a little party to reveal the sex of our baby, it attracted some attention online, to the extent that a magazine aimed at pregnant women picked up the story. Since the publication was available in many midwives' waiting rooms across the country, the idea caught on in a way I had never anticipated: now, everyone from friends to celebrities including Alec Baldwin, Kate Hudson and Jessica Alba throw them to celebrate the announcement of their newborn's gender.