The announcement said the Queen and members of both families are delighted with the news. Kate was scheduled to visit London's Hornsey Road children's center, which provides childcare and other services for local families.
Their choice of Thomas's Battersea in south London indicated that the Cambridges were settling into their Kensington Palace apartment, having moved recently from their Norfolk home Anmer Hall.
William, who quit his job as a pilot for the East Anglian Air Ambulance in July to become a full-time royal, is also due to attend the National Mental Health and Policing Conference in Oxford on Tuesday.
Trump administration wants to tie Harvey relief money to debt ceiling
We need to make sure that we can get to Texas the appropriate amount of money to rebuild the state. And I think as you know, that's our priority.
It is unlikely they will follow in the footsteps of William's great-great-great-great-grandmother Queen Victoria, who had nine children.
After being gifted a toy suitable for a newborn baby during an event in the country, she turned to Will and joked: "We will just have to have more babies!".
Regardless of whether the child is a boy or a girl, the baby will bump Prince Harry down the line of succession to fifth behind Charlotte.