Catherine and the Duke of Cambridge travelled to the Lindo Wing at St Mary's Hospital, Paddington, in central London on Monday morning.
Catherine has been on maternity leave since making a last royal visit to a charity lunch in London on 22 March.
The baby will be fifth in line to the throne and the Queen's sixth great-grandchild.
The birth will be announced with an email to the press and a celebratory tweet posted on the Kensington Palace Twitter feed.
There will also be the traditional custom of placing a framed paper proclamation on an ornate gold stand behind the iron railings of Buckingham Palace.
As with her first two children, Kate is hoping for a natural birth and does not know whether she is having a boy or a girl.
The duchess's pregnancy was announced in October.
As with her previous two pregnancies, Catherine, 36, has suffered from hyperemesis gravidarum, or severe morning sickness.
The condition affects about one in every 200 pregnancies and results in severe nausea and vomiting - with one of the main dangers being dehydration.
To become King or Queen as the third-born royal child is rare - and has yet to happen within the current House of Windsor.
But the third child of George III and Queen Charlotte, William IV, took on the task and ruled from 1830 to 1837.
The Hanoverian king acceded to the throne aged 64 when his older brother, George IV, died without an heir.
He became next in line when he was 62 and his other older brother, Frederick, Duke of York, died.
Read Full Story
Facebook
Twitter
Pinterest
Instagram
Google+
YouTube
LinkedIn
RSS