The Duchess of Cambridge entered a London hospital Monday in labour with what will be the third child for her and husband Prince William, Britain's Kensington Palace said.
The 36-year-old duchess traveled by car to the private Lindo Wing of St. Mary's Hospital in central London. The palace said she was in "the early stages of labour."
William and Kate married in 2011 and have two other children: Prince George, 4, and Princess Charlotte, who turns 3 next month. Both were born at the same hospital, as were William and his younger brother Prince Harry.
The baby, whose gender has not been announced, will be Queen Elizabeth II's sixth great-grandchild and fifth in line to the throne, after grandfather Prince Charles, father Prince William and the two older siblings.
The new arrival bumps Prince Harry to sixth place in the line of succession.
The 36-year-old duchess, formerly Kate Middleton, carried out her last official engagement on March 22 before going on maternity leave.
No exact due date has been given, with officials only saying that the baby was due in April.
As in her previous pregnancies, Kate suffered from hyperemesis gravidarum, a severe form of morning sickness. Officials announced both her previous pregnancies before the traditional 12-week mark because she was too unwell to attend public engagements.
Television crews, journalists and royal fans have set up camp outside the hospital for the "royal baby watch" since early April in anticipation of the arrival.
Many are betting on a traditional royal name for the baby, with bookmakers saying Mary, Alice, Arthur and James are the most popular guesses.