It is a historic moment for the British monarchy — the couple's first child will become third in line for the British throne, after Prince Charles and William, and should eventually become king or queen.
William and Kate entered St. Mary's Hospital in central London through a side door early Monday morning, avoiding the world's media. Palace officials confirmed her arrival about 90 minutes later.
Royal officials said they traveled by car, without a police escort, just before 6 a.m. Kate — also known as the Duchess of Cambridge — is expected to give birth in the private Lindo Wing of the hospital, where Princess Diana also gave birth to William and his younger brother, Prince Harry.
"Things are progressing as normal," the couple's spokesman said.
The baby is in line to become Britain's monarch and be recognized as head of state in 15 other countries, including Canada and Australia, playing a leading role in charting the future of the Commonwealth.
But little else is known, including how the baby will arrive — it is not clear if Kate will have a natural birth or deliver by a planned cesarean section — its gender and its name.
Prince Charles, about to become a grandfather, told well-wishers early Monday afternoon that he had not received any updates about Kate's condition.
Royal watchers must wait to be told of the baby's arrival from the palace, which is planning to reveal the news through a mixture of tradition and social media.
