Nineteen months after a modern fairy tale of a wedding turned the duke and duchess of Cambridge into the world’s most famous newlyweds, the royal juggernaut known as Will and Kate is officially expecting what may very well turn out to be the world’s most famous baby.

Britain’s great and small toasted the eagerly anticipated arrival of the royal child, who will be third in line to the throne and a grandchild of the late Diana, Princess of Wales. But the news of pregnancy was immediately tempered by the hospitalization of the mother-to-be because of an unusually acute case of morning sickness.

The duchess, known as Kate Middleton before her marriage, was suffering from hyperemesis gravidarum, prompting her admission to London’s exclusive King Edward VII Hospital for “several days” of medical care to ensure proper nutrition. Royal officials warned that the duchess would “require a period of rest” after her discharge.

Across Britain, there were expressions of concern for her health, laced with hopes that her illness — severe vomiting that can be accompanied by low blood pressure and other symptoms — would not foreshadow months of difficult pregnancy. The Telegraph reported that her relatively rare condition had forced the royal family to accelerate the announcement, which was initially planned after she passed the 12-week mark. Royal officials would not disclose how far along the duchess is in her pregnancy.

“It’s a serious condition in that it makes you feel awful — the constant vomiting can leave you very dehydrated,” said Tim Draycott, a consultant obstetrician for the Royal College of Obstetricians and Gynaecologists and a specialist on Catherine’s illness. “But it’s a condition that has few long-term consequences for the baby. About 30 percent of women get morning sickness; only about 1 percent of women get symptoms like Kate Middleton. Hyperemesis is slightly more common in twin pregnancies.

“Treatment for hyperemesis includes intravenous fluids, anti-nausea medication and extra vitamins. It’s most common from six weeks onwards and starts to settle down around 12 to 13 weeks of pregnancy.” 

Draycott added: “Poor her.”

The pregnancy announcement, which brought well wishes from the White House and the crowned heads of Europe, heralds what would be a landmark birth in Britain of a 21st-century monarch-to-be in every sense. The child would be the rare product of a union between a celebrated British royal and a so-called “commoner” who is the great-great-granddaughter of a coal miner. The royal couple’s offspring would be the first born after a historic change was set in motion last year to eliminate the tradition of male hereditary precedence to the throne in Britain and commonwealth nations, where Queen Elizabeth II is head of state. So if the baby — who would leapfrog William’s brother, Prince Harry, upon birth — is a girl, a younger brother would not be able to leapfrog her.

A nation only just emerging from a double-dip recession and perhaps slouching toward a third celebrated the needed dose of good news before the holidays. Tabloids screamed the news in apocalyptic headline type. The BBC News Channel broke in with an urgent bulletin and broadcast running coverage. A rash and swell of British and foreign media flocked to the entrance of King Edward VII Hospital for what appeared likely to be a round-the-clock camp out.

Even Prime Minister David Cameron could not contain himself, saying he blurted the news out during an official meeting with aides after receiving a “little note.”

“It’s absolutely wonderful news, and I’m delighted for them and I’m sure they will make absolutely brilliant parents,” Cameron said. “I’m sure people round the country will be celebrating with them tonight.”

Speculation had grown in recent weeks that the couple might be expecting, though the rumor mill appeared driven more by desire and anticipation than insider knowledge. Tabloids had taken to scouring every photo of the duchess for evidence of a “royal bump.” On Wednesday, the saucy Daily Mail reported that the duchess’s new chic coiffure must surely be a harbinger of pregnancy: “If she is keeping a Very Important Secret, then that demure long fringe is perfect for hiding behind.”

The hawk-eyed could detect perhaps more reliable signs of royal fecundity. On a recent tour of Asia, the duchess eschewed wine for water during a state dinner, prompting torrid rounds of baby speculation. And last week, a young mother in Cambridge handed Prince William — a Royal Air Force search-and-rescue pilot — a handmade onesie with a helicopter and the words “Daddy’s little co-pilot” embroidered on it. According to Britain’s ITV television network, William promptly placed the suit in the care of an aide, quipping, “I’ll keep that.”

As the couple steamrolled toward their second wedding anniversary, royal-watchers pointedly noted that Queen Elizabeth II gave birth to Prince Charles almost a year to the day after her wedding. The late Diana produced an heir — the now-expectant father — after 11 months of marriage.

In the Queens Park neighborhood of North London — known as “the Nappy Valley” because of its proliferation of families with babies — a parade of young couples wished Will and Kate well. “It’s perfect, why wait?” said Miranda Little, 36, a full-time mother pushing her 10-month-old son in a stroller. “Get on with it, give the people what they want.”

Yet in a country rich in gross national sarcasm, the blogosphere also rang out with priceless missives of the sort posted on the mothers’ chat site Mumsnet by someone going by the handle HullyEastergully: “Oh how lovely! Another little privileged sprite to ponce off the taxpayer! Joy to the world.”

For the House of Windsor at least, the royal couple remained the gift that keeps on giving, and just in time for the holidays. Their wedding, watched by 2 billion people on six continents, helped usher in a renaissance of a family once stained by constant scandal. Polls show that support for the monarchy has never been higher in the modern era. After recent doses of racy press — including a topless photo of the duchess and photos of Prince Harry playing naked pool — the pregnancy offers the family a more traditional talking point.

Royal-watchers say the birth could mark the evolution of the duke and duchess’s roles in the public eye, with Prince William in particular expected to take on an ever-expanding list of official duties next year.

“This is the Christmas present people have been hoping for, and what a royal family is all about — renewal,” said Robert Lacey, royal biographer and author of “The Queen: A Life in Brief.” “There is nothing like youth to revive monarchy. This announcement is about the next stage to come, with the young couple more and more in the public eye as the next generation of royal family.”

Eliza Mackintosh contributed to this report.