Kate Middleton has been admitted to the hospital to give birth, according to a Kensington Palace spokesman.
"Her Royal Highness The Duchess of Cambridge has been admitted this morning to St. Mary's Hospital, Paddington, London in the early stages of labour. The Duchess travelled by car from Kensington Palace to the Lindo Wing at St Mary's Hospital with The Duke of Cambridge," the palace said in a statement today.
Middleton was admitted shortly before 6 a.m. today with The Duke by her side, and she is progressing normally, the palace said. There is no suggestion that she was induced.
Regardless of the baby's gender, Prince William and Kate Middleton's firstborn will be third in line to the British throne, after grandfather Prince Charles and father Prince William.
Kate and William have chosen to be surprised and do not know the sex of the baby, according to royal sources.
The British Parliament is changing a 300-year-old law so that the baby will be the heir to the throne, whether it's a boy or girl. Under plans to change the rules of succession, if the baby is a girl, she will make history as the first girl to keep her place in line even if she later has a brother.
The reigning monarch, Queen Elizabeth II, was only eligible to rule because her father had no male children.
Elizabeth has been on the throne 61 years. This baby will be the queen's third great-grandchild.
Kate is expected to give birth in the Lindo Wing of St. Mary's Hospital in Paddington, where Diana had William and Harry.
The formal birth announcement will come after the queen, royal family and Middletons have heard the good news, royal sources say.
The announcement will follow the royal tradition of posting a paper announcement signed by the doctors on an easel outside Buckingham Palace. The announcement is expected to include the baby's gender, weight and time of birth.
This old-fashioned announcement will be accompanied by a few more modern methods. The palace is also expected to announce the birth through Twitter and their website.
Prince William was the first royal heir to be born in a hospital instead of in the palace. The first sighting of him and Prince Harry was when their parents exited the hospital with the new princes in tow. William and Kate are expected to do the same with their new arrival.
It's unknown when the baby's name will be revealed. Harry's name was announced the day he left the hospital, but several days passed before William's name was made public.