Prince William and his wife Catherine do not know the sex of their baby, which is expected in mid-July and will be born by natural delivery, palace sources said on Wednesday.
The baby, which will be third in line to the throne, will be born in the same London hospital where princess Diana gave birth to William and his younger brother Harry.
While the monarchy is on Twitter, the birth will be announced to the world in the traditional method -- a notice posted on an easel in the forecourt of Buckingham Palace.
William, the Duke of Cambridge, will take paternity leave from his role as a Royal Air Force search and rescue pilot.
He is expected to be at his wife's bedside for the birth. The new royal baby will be born in the private Lindo wing of St Mary's Hospital in Paddington, central London, where William was born in 1982 and his brother Harry in 1984.
The updates from palace sources on events surrounding the birth are the first confirmed details about the new royal arrival, due within weeks.
William and Kate want the sex of their baby to be a surprise when it arrives.
"The duke and duchess do not know the sex of their baby and they've decided not to find out beforehand," a source said.
Kate said on a public engagement in March that she did not know whether she was carrying a boy or a girl.
If the baby is a girl, it cannot be overtaken in line to the throne by any future younger brothers, following a change in the succession laws hastily agreed by the 16 Commonwealth realms, which include Britain, Canada, Australia and New Zealand.