Prince William's wife Catherine spent the night in hospital where she is being treated for acute morning sickness following the announcement that the couple are expecting their first child.
The family revealed the news earlier Monday, ending fevered speculation about a baby destined to become Britain's monarch whether it is a boy or a girl.
But St James's Palace also revealed that the former Kate Middleton, 30, was in hospital suffering from "hyperemesis gravidarum".
Queen Elizabeth II and the rest of the royal family were "delighted" by the news of the pregnancy while William -- the son of the late Princess Diana and the heir to the throne Prince Charles -- had visited her in hospital, the palace said.
The pregnancy adds to resurgent support for the monarchy after the pomp of William and Catherine's wedding in April 2011 and the Diamond Jubilee celebrations to mark the queen's 60th year on the throne in June this year.
"Their Royal Highnesses The Duke and Duchess of Cambridge are very pleased to announce that The Duchess of Cambridge is expecting a baby," St James's Palace said in a statement, using the couple's official titles.
The baby is set to be third in line to the British throne after Charles and William, irrespective of its gender, following moves by the government to change ancient succession laws giving priority to male children.
The palace said the couple were only "recently" aware that Catherine was pregnant. The Press Association reported that the pregnancy has not passed the 12-week point and that the announcement was prompted by her medical condition.
The palace said Catherine was admitted on Monday afternoon to the King Edward VII Hospital in central London with hyperemesis gravidarum, which it defined as "very acute morning sickness", which requires extra hydration and nutrients.
"As the pregnancy is in its very early stages, Her Royal Highness is expected to stay in hospital for several days and will require a period of rest thereafter," the statement said.
William, also 30, visited his wife before leaving to a flurry of flashbulbs at around 8:20 pm (2020 GMT).
British media said the couple had been staying at her parents' house in Berkshire, west of London, and travelled by car to the hospital.
The queen, her husband Prince Philip, William's father Prince Charles, Charles's wife Camilla, William's brother Prince Harry "and members of both families are delighted with the news," the palace said.
Prime Minister David Cameron said the royal couple would make "wonderful parents" and that "people round the country will be celebrating with them tonight."
William's uncle Earl Charles Spencer -- the brother of Diana who won praise for his emotional eulogy at her funeral after she died in a car crash in Paris in 1997 -- said he was "thrilled for them both".
US President Barack Obama and his wife Michelle, who met the royal couple during a state visit to Britain last year, sent their congratulations on the "welcome news", the White House said.
Britain's newspapers on Tuesday celebrated the "delightful" news.
Popular tabloid The Sun ran with "Kate Expectations" as its front-page headline while The Daily Telegraph said the news was cause for national celebration.
"Who would not be delighted at the prospect of a mother’s first child, especially a mother who has won affection with her natural beauty and straightforward character?" said its editorial.
Catherine, a "commoner" whose parents are self-made millionaires from a party supplies business, met William at St Andrews University in Scotland in 2001.
Speculation about a royal pregnancy intensified last Wednesday when the couple were given a baby's romper suit during their first visit together to Cambridge, the historic English university city which is home to their dukedom.
William laughed and said "I'll keep that" after accepting the tiny hand-stitched outfit with the words "Daddy's little co-pilot", a reference to his job as a Royal Air Force search and rescue helicopter pilot.
The baby news comes just over one year since Commonwealth nations agreed to scrap centuries-old laws barring first-born daughters from inheriting the British throne if younger male heirs were available.
Deputy Prime Minister Nick Clegg said Monday that Britain was currently in the process of overhauling the 1701 Act of Settlement.