The royal couple had to spill the beans that Kate was preggers after she became violently ill and Prince William accompanied her in a drive from her family’s home in Berkshire to a London hospital.
There Kate was diagnosed with Hyperemesis Gravidarum, a severe form of morning sickness that affects one in 50 women.
A worried-looking William later left King Edward VII Hospital and hopped into a waiting car without a word to the army of reporters camped outside.
He had only told the queen, his dad, Prince Charles and his brother Prince Harry the news Monday morning when Kate’s hospitalization forced the early public announcement.
But the former Kate Middleton’s parents were in on the secret because she’d started throwing up — and had been unable to keep down any food or liquids — while spending the weekend with them.
Worried, William consulted some doctors and made the decision to get his beloved to a hospital — but by car, so as not to attract too much attention, British media reported.
Kate, 30, will be hospitalized for several days “and will require a period of rest thereafter,” the statement from St. James’s Palace read.
The pregnancy is in its “very early stages,” the palace said. “The entire royal family “is delighted with the news.”
British media is reporting that Kate is between six and 12 weeks along.
Left untreated, Kate would be at risk of dehydration. But now that she’s getting the care she needs, doctors expect her to return to her royal routine within a few weeks.
Kate Middleton fueled baby rumors during the Opening Ceremony of the London 2012 Olympic Games in July.
William, 30, is second in line for the throne after his father, Prince Charles. That means his spawn, be it a girl or a boy, would be third in line to become monarch, under the royal rules.
The ancient rules of succession were changed last year to allow the firstborn royal child — girl or boy — to ascend to the throne. Previously, a boy was heir to the throne even if his sister was born first — as Charles’ older sister Princess Anne was.
The baby would also be the first grandchild for Prince Charles and the third great-grandchild of the ruling monarch, Queen Elizabeth.
President Obama congratulated the parents-to-be.
“On behalf of everyone here in the White House, beginning with the President and first lady, we extend our congratulations to the Duke and Duchess of Cambridge on the welcome news this morning out of London that they are expecting their first child,” he said in a statement.
New York’s expat British community was thrilled by the happy news.
Haley Simmonds, who works as a hostess at Tea & Sympathy in the West Village, suggested William name the baby after his wild man brother or his departed mother.
“Harry if it’s a boy — that’s a good name,” said Simmonds, 27, who hails from London. “Diana would be nice” if it’s a girl.
“It’s exciting,” added 24-year-old Max Turpin, who is from Reading. “If there’s another reason to have a party in the streets of London, I’m all for it.”
Kate Middleton and Prince William wave to fans during a carriage procession to Buckingham Palace after their marriage at Westminster Abbey on April 29, 2011.
Turpin said William should name his son after a British soccer star — and not David Beckham.
“His name would be Wayne because he'd be named after the greatest football player that ever lived, Wayne Rooney,” he said.
British Prime Minister David Cameron tweeted his congratulations.
“I’m delighted by the news that the Duke and Duchess of Cambridge are expecting a baby,” he wrote. “They will make wonderful parents.”
And William's uncle, Earl Spencer, brother of William’s late mother Princess Diana, said the pregnancy was "wonderful news and I am thrilled for them both.”
The intense speculation about when the duke and duchess of Cambridge might have a child began shortly after the young couple tied the knot on April 29, 2011, in a lavish Westminster Abbey wedding.
William was conceived four months after Charles and Diana married. And every time the duchess appeared in public it seemed the nation’s eyes were on her belly.
Except, of course, when they were distracted by topless photos of Kate taken this summer while she was on vacation with William in France.
The couple stoked speculation that Kate might be pregnant on an official trip to Singapore in September when she and the prince clinked glasses of ice water rather than champagne at a reception in honor of Queen Elizabeth’s Diamond Jubilee.
That would have been about 12 weeks ago.
Then last week, Prince William got the rumor mill going again when a well-wisher handed him a onesie with a picture of a helicopter and the words, “Daddy’s little co-pilot.”
“I’ll keep that,” the grinning prince said.
In recent days, Kate gave no sign of being with child as she kept up a busy schedule of royal appearances.
In fact, Kate was just photographed on Friday bounding across a field — dressed in a green plaid dress and black high-heeled boots — playing field hockey with students at her former school.