Britain: Duke and Duchess of Cambridge do not know sex of royal baby
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 Buckingham Palace forecourt.
William, the Duke of Cambridge, will take paternity leave from his role as a Royal Air Force search and rescue pilot, though it is not known how long the duchess will take off royal duties.
William is expected to be at his wife's bedside for the birth.
The 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 are the first confirmed details about the birth, expected within weeks.
William and Kate want the sex of their baby to be a surprise when it arrives.
Kate, the duchess of Cambridge, made her final public appearance before going on unofficial maternity leave on Saturday, at the annual parade marking Queen Elizabeth II's birthday.
The duchess shared a carriage with her husband's brother, Prince Harry, and her step-mother-in-law the duchess of Cornwall, as they drove to the military parade, known as Trooping the Colour, in central London.
She wore a pale pink Alexander McQueen coat and matching hat, and looked relaxed as she smiled and waved at the crowds gathered. She is due to give birth in mid-July.
The queen's husband Philip, the duke of Edinburgh, remained in hospital where he is recovering from abdominal surgery.
About 1,000 soldiers took part in the event, with military bands and precision marching. Prince William, Kate's husband, took part in the parade on horseback, as did his father Prince Charles.
The queen took a 41-gun salute on Horse Guards Parade, after which the royals returned to Buckingham Palace to watch the traditional RAF flypast from the balcony.
The 87-year-old queen celebrated her actual birthday on April 21.
Voice of Russia, AFP, dpa