Kate, the Duchess of Cambridge, gave birth Monday to a healthy baby boy — a third child for Kate and Prince William and fifth in line to the British throne.
The couple's Kensington Palace office announced news of the birth around lunchtime, about five hours after the 36-year-old duchess and her husband traveled by car from their Kensington Palace home to the private Lindo Wing of St. Mary's Hospital in central London.
The palace said the baby prince was born at 11:01 a.m. (1001 GMT; 6:01 a.m. EDT) and weighed in at eight pounds, seven ounces (3.8 kilograms). Prince William was in attendance, and the palace said mother and child were both doing well.
She left the hospital about seven hours later, stopping at the steps of the hospital to greet the well-wishers with her new child in her arms and William by her side.
"The queen, the Duke of Edinburgh, the Prince of Wales, the Duchess of Cornwall, Prince Harry and members of both families have been informed and are delighted with the news," the palace said in a statement.
Mixing tradition and modernity, the news was announced by the palace on Twitter and also proclaimed in the forecourt of Buckingham Palace through a framed notice perched on a golden easel.
Tony Appleton, a town crier from southeast England, showed up in full regalia to declare the newborn prince's birth outside the hospital. The words "It's a boy" flashed in lights around the top of London's BT Tower, which can be seen for miles around.
More ceremonial celebration is to come, including the pealing of bells at Westminster Abbey and a gun salute in London's Hyde Park.
The baby is a younger brother to 4-year-old Prince George and Princess Charlotte, who turns 3 next week. Both were born at the same hospital, as were William and his younger brother, Prince Harry.
They came to the hospital later Monday to meet their new baby brother. George wore the navy blue shorts and sweater of his school uniform and ignored the waiting media and well-wishers. Charlotte gave the crowd a wave as the family entered.
The infant's name, which has been subject to a flurry of bets, hasn't been announced. Arthur and James are among bookmakers' favorites for the new prince, whose full title will be His Royal Highness, Prince (Name) of Cambridge.
Monday is St. George's Day, England's national day, but the baby is unlikely to be given the name since his older brother already has it.
The new arrival is Queen Elizabeth II's sixth great-grandchild and bumps Prince Harry to sixth place in the line of succession. The baby is fifth in line, after grandfather Prince Charles, father Prince William and his two siblings.
Charlotte is the first royal daughter to stay ahead of a younger brother in the line of succession. Before the rules were changed in 2012, male heirs took precedence.
Kensington Palace announced in September that Kate was pregnant with her third child. As in her previous pregnancies, the duchess suffered from hyperemesis gravidarum, a severe form of morning sickness.
Officials announced her previous pregnancies before the traditional 12-week mark because she was too unwell to attend public engagements. This time around, it kept her from taking Prince George to his first day of school.
The 36-year-old duchess, formerly Kate Middleton, nevertheless kept up a busy schedule of royal duties during her pregnancy, including a visit with William to Scandinavia. She carried out her last official engagement on March 22 before going on maternity leave.
The birth was overseen by a team of doctors including consultant obstetrician Guy Thorpe-Beeston and consultant gynecologist Alan Farthing — who were also called in for the births of George and Charlotte — as well as the hospital's midwives.
Television crews, journalists and royal fans had set up camp outside the hospital for the "royal baby watch" since early April in anticipation of the arrival.
John Loughrey, a veteran royal-watcher who has been camped outside the hospital for two weeks, said the baby would be "very good for our country and of course Her Majesty the queen."
"I'm so pleased it's St. George's Day," he said before the birth was announced. "St. George himself would be very pleased if the baby's born today."
William and Catherine celebrated some other family milestones in the past year. Charlotte began attending nursery school in January. To mark the occasion, the Palace released special photos Catherine took of the princess before her first day at Willcocks Nursery School in London.
And George, who turns 5 in July, attended his first day of school in September. On the big day, William walked his son hand-in-hand to the entrance of Thomas's Battersea in London.
NBC's Liz Lane and the AP's Jill Lawless contributed to this story.