It's thought that the news was announced earlier than planned, after Kate was confirmed to be suffering with acute morning sickness for the third time, leading her to cancel several public appearances and miss Prince George's first day of school.
Whilst no official timeline has been given, some royal fans have picked up on a subtle hint as to how far along the Duchess of Cambridge might be, after her husband was overheard letting slip the tiniest of indications at a recent engagement.
Speaking with an audience member at the National Mental Health and Policing conference in Oxford, who had congratulated him on the happy news, William said: 'Thank you, it's very good news. There's not much sleep going on at the moment!'
And then – here's the clincher – he added: 'It was a bit anxious to start with, but [we can] start celebrating this week.'
Expectant parents usually wait until the 12 week mark – the end of the first trimester – to announce their pregnancy, or 'start celebrating', as Wills put it. So, if the royals have stuck to tradition, we can expect to welcome a new royal baby and fifth in line to the throne sometimes in March of next year!
Of course, it's possible that Kate's onset of Hyperemesis Gravidarum could have developed in the very earliest stages of her pregnancy, so we've no certain way of knowing how long the Duchess has been suffering with the intense form of morning sickness.
The official statement from the palace announcing the pregnancy said: 'Their Royal Highnesses The Duke and Duchess of Cambridge are very pleased to announce that The Duchess of Cambridge is expecting their child. The Queen and members of both families are delighted with the news.'
'As with their previous two pregnancies, The Duchess is suffering from Hyperemesis Gravidarum. Her Royal Highness will no longer carry out her planned engagement at the Hornsey Road Children's Centre in London today. The Duchess is being cared for at Kensington Palace.'