Kate, Duchess of Cambridge, will give birth once again at Lindo Wing at St Mary's Hospital in west London and is expected to introduce the new-born on the same steps outside as Prince George back in 2013.
"The royal couple do not know the sex of their child," Kensington Palace said, quashing all speculation that Kate had been buying pink baby clothes in preparation of a baby girl.
It is believed her due date is in the second half of this month and may still coincide with the May 7 General Election in case of a delayed pregnancy.
Royal officials said the couple are "hugely grateful" for the warm wishes they have already received.
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During the birth of her second child, Kate will again benefit from the private wing being based within a National Health Service (NHS) hospital with facilities for premature babies, and access to hospital specialists.
Prince George was delivered there weighing 8lb 6oz on July 22, 2013.
The Lindo wing describes itself as having offered "leading private obstetric and neonatal care for 60 years".
The Royal Mint has announced it will mark the birth of the Duke and Duchess of Cambridge's new child in the same way it commemorated the arrival of Prince George in 2013.
Parents of newborns who share a birthday with the new prince or princess will have to register the birth of their child on The Royal Mint's Facebook page to receive the commemorative gift.
Prince William and Kate's second baby will become fourth in line to the throne, behind older brother Prince George, who is 18 months old.
If the new baby is a girl, she will follow Prince George and become fourth in line to the throne and will not be overtaken by any future younger brothers.
The Act was passed by Parliament in 2013, but all the countries in which the Queen is head of state had to pass any necessary legislation before it took effect.