It all started when actress and comedian Miranda Hart, who served as host for this year's charity broadcast, allowed one young guest to make a special request of the prince. Following the announcement from Kensington Palace that Meghan Markle would be joining the royal family for Christmas this year at Sandringham, the bride-to-be attended another firm royal family fixture for the festive season this week - the Queen's annual Christmas lunch.
Self-Portrait is a label that the Duchess of Cambridge has worn in the past, so Kate knows that it's sure to be a hit with royal fashion fans, as well an acceptable label to wear for an event connected to the monarchy.
Charlotte is seen wearing a pair of blue Mary Janes while her brother wears black leather loafers with long socks - although one has fallen partway down his ankle. Prince William is seen dressed in a royal blue suit, with a light blue, patterned tie.
In 2015, the family posted an outdoors picture of themselves on the card, with blue being the colour of choice then as well.
One said: "Royal baby definitely a boy!"
Since Kate is due in April and the photo was taken in July, that would have meant that she may not have even known that she was pregnant when the picture was taken, let alone what the sex of her baby is.
The Cambridges' photo was shared on the official Kensington Palace Twitter account with the short statement: "The Duke and Duchess of Cambridge are pleased to share a new photograph of their family".
Yesterday, odds were slashed on the abby being named Alice or Arthur after bookies received a flurry of bets.