Media playback is unsupported on your device Media caption The news was announced earlier than planned due to Catherine's illness, as Nicholas Witchell reports 
 
 The Duchess of Cambridge is pregnant with her second child. 
 
 Kensington Palace said that - as when the duchess was pregnant with Prince George - she was suffering from very acute morning sickness and was being treated by doctors at the palace. 
 
 Prince William and Catherine's second baby will become fourth in line to the throne, behind older brother Prince George, who is 13 months old. 
 
 The palace said the Queen and both families were delighted with the news. 
 
 Prime Minister David Cameron, Labour leader Ed Miliband and the Archbishop of Canterbury were among those to offer their congratulations to the couple. 
 
 Early announcement 
 
 Image copyright Reuters 
 
 By Peter Hunt, BBC News royal correspondent 
 
 For a second time, William and Kate have been forced to announce a pregnancy before the duchess passed the significant 12-week milestone. 
 
 For a second time it's because Kate is suffering from an acute form of morning sickness - though this time she's being treated behind palace walls and not at a private hospital with representatives of the world's media gathered outside. 
 
 The pregnancy has generated international excitement and will continue to do so but this impending birth will lack the constitutional significance of Prince George's arrival. 
 
 As things stand, the as yet unborn girl or boy isn't destined to be a monarch. She or he will occupy the same position as Prince Harry once did. His mother, Diana, used to call him "the back up". 
 
 Monday's announcement has performed one unintended but useful role for the Windsors. Headlines about the Queen's view of Scottish independence will be replaced by extensive coverage of a royal birth next year. 
 
 Read more from Peter 
 
 Overseas tour 
 
 The duchess's first pregnancy was revealed when she was just a few weeks pregnant with Prince George after she was admitted to hospital suffering from severe morning sickness in December 2012. 
 
 The sickness - called hyperemesis gravidarum - is a condition that may require supplementary hydration, medication and nutrients. 
 
 It affects 3.5 per 1,000 pregnancies, causes severe vomiting and can lead to dehydration, weight loss and a build-up of toxins in the blood or urine, called ketosis. 
 
 Media playback is unsupported on your device Media caption A beaming Prince William arrives at Oxford University to cheering crowds 
 
 Catherine had been due to join Prince William in formally opening a £21m China study centre at Oxford University but the palace said her morning sickness meant that she would no longer be attending. 
 
 The Duke of Cambridge arrived at the new study centre at about 13:30 BST, where he was presented with a bouquet of flowers by a young girl. 
 
 He told a well-wisher who sympathised with Catherine's condition that she may be over the worst of the acute morning sickness in a "few weeks' time". 
 
 Kensington Palace said the duchess's attendance at future events would be decided on a "case-by-case" basis. 
 
 Image copyright Getty Images Image caption The duke was presented with flowers by five-year-old Beth Barton when he arrived at the university 
 
 Image copyright Reuters Image caption The duke and duchess recently attended WW1 commemorations 
 
 Image copyright PA Image caption They visited New Zealand and Australia with Prince George earlier this year 
 
 She is scheduled to be among the guests at the opening ceremony of the Invictus Games in London on Wednesday evening along with Prince William, Prince Harry and the Prince of Wales and Duchess of Cornwall. 
 
 On Thursday she is due to attend a Drumhead service at Lee Valley Athletics Centre with William and Harry, and later with William to watch Invictus competitors taking part in matches and heats. 
 
 The duchess is due to make her first official solo overseas tour, to Malta for two days, from 20 September. 
 
 'Big brother' 
 
 The baby announcement came after the couple's first child, Prince George, had his first birthday on 22 July. 
 
 Prime Minister David Cameron said: "Many congratulations to the Duke and Duchess of Cambridge. I'm delighted by the happy news that they're expecting another baby." 
 
 Mr Miliband said: "Fantastic to hear that Prince George will soon be a big brother. Congratulations to the Duke and Duchess of Cambridge on their happy news." 
 
 Media playback is unsupported on your device Media caption How the world broke the news of the next British royal baby 
 
 Deputy Prime Minister Nick Clegg said the pregnancy was "fantastic news". 
 
 Scottish First Minister Alex Salmond said: "This is very happy news for any couple, and on behalf of the people of Scotland, I am delighted to send our best wishes to the countess and our hearty congratulations to the royal couple." 
 
 The duchess is known as the Countess of Strathearn in Scotland. ||||| By Radar Staff 
 
 With the world buzzing about Kate Middleton‘s pregnancy, medical experts say the fact that she is experiencing hyperemesis gravidarum, or severe nausea, could very well signal that the Duchess of Cambridge is expecting twins. 
 
 With Prince William at her side, Kate remains in a London hospital with the rare condition that impacts 2 percent of pregnant women, ABC News reported, leading to weight loss and dehydration. 
 
 PHOTOS: The 11 Most Adorable Celebrity Babies Of The Year 
 
 Medical experts told the network the condition is linked with women having twins. 
 
 Dr. Ashley Roman, of the at New York University Langone Medical Center, said that a study of the condition revealed women who are carrying twins held a 7.5 percent higher risk of experiencing hyperemesis gravidarum. In some cases with the condition, a female child was more likely, the network reported. 
 
 Roman said that the condition shouldn’t impact the baby’s health. She told the network that Kate’s symptoms — nausea and vomiting — are “traditionally thought [of as] a sign of a healthy pregnancy.” 
 
 PHOTOS: Duchess Of Cambridge Visits A Mosque 
 
 Most of the cases conclude 16 or 20 weeks into the pregnancy, Roman said, adding that in scarce cases, it can last the entire nine months. 
 
 Ohio OB/GYN Dr. Nancy Cossler told ABC News Kate will likely have to endure “constantly feeling sick and puking” as she gets the illness out of her system. 
 
 “The biggest problem with this is how it interferes with your life,” Cossler said. 
 
 PHOTOS: Royal Bombshell! Kate Middleton’s Sexiest Looks Ever 
 
 Elsewhere, gamblers were wagering early and often as to what the royal heir’s name would be, with the same batch of names at the top of the favorites board. 
 
 At the English casino Ladbrokes, Elizabeth was the 8/1 favorite, with Frances at 10/1 and Diana at 12/1 odds if the child is female; if male, the casino had John or Charles at 10/1, James at 12/1, Philip or George 14/1, Edward at 16/1 and Andrew at 20/1. 
 
 PHOTOS: Kate Middleton Makes Vanity Fair’s International Best-Dressed List 
 
 Ladbrokes’ Jessica Bridge explained why Elizabeth was the favorite of all prospective names. 
 
 “Kate and Wills pulled the wool over our eyes, but now attention turns to the name of the baby and it looks like the couple will stick with a traditional name,” Bridge said. “If they’re expecting a girl, who better to name her after than Her Majesty?” 
 
 The casino William Hill had Frances — the name of William’s grandmother on his mother’s side — and John as 9-1 favorites, with Charles, George and Victoria, all paying out at 10-1. As for gender, Hill has odds of 10/11 for either male or female. 
 
 For those who like to play the longshots, Paddy Power, a Dublin casino, had Camilla, as in Camilla Parker Bowles, at 66-1 and or Fergie, as in Sarah Ferguson, at 250-1. 
 
 RELATED STORIES: 
 
 A Royal Baby? Kate Middleton’s Close Pal Says Announcement Will Be Made Next Month 
 
 Kate’s Topless Photographer Identified! Name Of Pap Who Snapped Middleton Given To Authorities 
 
 Prince Harry’s ‘Drunken Fumble’ Girl Is Negotiating An Exclusive TV Interview 
 
 Pippa Middleton Dating Rich Brother Of British Reality Star ||||| Image copyright Getty Images Image caption The couple took their two children, George and Charlotte, on an official visit to Poland in July 
 
 The Duke and Duchess of Cambridge are expecting their third child, Kensington Palace has announced. 
 
 The Queen and both families are said to be "delighted with the news". 
 
 As with her previous two pregnancies, the duchess, 35, is suffering from hyperemesis gravidarum, or severe morning sickness. 
 
 She pulled out of an engagement at the Hornsey Road Children's Centre in London, which had been planned for Monday afternoon. 
 
 Catherine is being cared for at Kensington Palace, the statement said. 
 
 The duke and duchess have a son, George, who is four, and a daughter, Charlotte, aged two. 
 
 With the previous two pregnancies, the couple announced them before the 12-week mark - when most women have their first scan - because of the duchess being unwell with hyperemesis gravidarum. 
 
 Her first pregnancy was revealed when she was just a few weeks pregnant with Prince George after she was admitted to hospital in December 2012. 
 
 Media playback is unsupported on your device Media caption Prince Harry gives a thumbs up to the royal baby news 
 
 The duchess's second pregnancy with Princess Charlotte was announced in September 2014, when she was treated at the palace for the condition. 
 
 Hyperemesis gravidarum affects about one in every 200 pregnancies and results in severe nausea and vomiting - with one of the main dangers being dehydration. 
 
 'Shaping future of monarchy' 
 
 Image copyright PA 
 
 By Peter Hunt, BBC royal correspondent 
 
 Once again Prince William and his wife - who are very focused on being in control - have been thwarted. 
 
 And once again, it's due to circumstances outside their control. 
 
 The couple have been forced to make the announcement at a time not of their choosing - and while the duchess is still in the early stages of her pregnancy - because she is suffering from very acute morning sickness. 
 
 They were poised to take on more royal duties. They are now preparing to welcome another addition to their family. 
 
 An addition that will attract considerable global interest. The child's grandmother is the late Diana, Princess of Wales. 
 
 This princess or prince is unlikely to be crowned monarch. As things stand, that future awaits their brother, Prince George. 
 
 So there is no constitutional significance to the birth next year. 
 
 But an ancient institution that already appears pretty secure has just been further buttressed. 
 
 The three Cambridge siblings will fashion the future of the British monarchy well into the 21st Century. 
 
 Read more from Peter 
 
 The BBC's royal correspondent Nicholas Witchell said the announcement comes at the start of a "significant week" for the family "because Prince George is due to start at big school." 
 
 "Presumably his mother would be keen to take him to that, [but] whether she is going to be well enough to do that remains to be seen," he told BBC News. 
 
 "It had also been expected that the Duke and Duchess of Cambridge would be taking a foreign trip this autumn," he added. 
 
 "Whether they will be able to do that or whether the duchess will be well enough to do that also remains to be seen." 
 
 The expected child will become the fifth in line to the throne behind Prince Charles, Prince William, Prince George and Princess Charlotte. 
 
 A change - which stops royal sons taking precedence over their female siblings in the line of succession - came into force in March 2015. 
 
 The child will be the Queen's sixth great-grandchild. 
 
 The last third-born monarch 
 
 Image copyright Reuters 
 
 To become King or Queen as the third-born royal child is rare - and has yet to happen within the current House of Windsor. 
 
 But the third child of George III and Queen Charlotte, William IV, took on the task and ruled from 1830 to 1837. 
 
 The Hanoverian king acceded to the throne aged 64 when his older brother, George IV, died without an heir. 
 
 He became next in line when he was 62 and his other older brother, Frederick, Duke of York, died. 
 
 Arriving in Manchester for a royal visit, Prince Harry - who will drop to sixth in line to the throne when the child is born - said the news was "fantastic" and he was "very, very happy". 
 
 Clarence House has tweeted on behalf of the Prince of Wales and Duchess of Cornwall to say they are "delighted". 
 
 Prime Minister Theresa May has tweeted her congratulations to the couple, calling it "fantastic news".
– Royal-watchers are ravenous for news about the forthcoming progeny of Prince William and wife Kate, so E! and other sites take a look at what life will be like for the soon-to-be third in line to the British throne: He or she will be welcomed with tolling bells, cannons, and bonfires. Yes, really. His or her last name will be the same as William's. Which is Mountbatten-Windsor, for those not in the know. If she's a girl, her name will probably include "Diana." But it most likely won't be her first name, just one of many, the Atlantic Wire points out. Europe's largest betting firm is already taking bets, reports People. Kate will probably breastfeed the little one. That might seem obvious, but up until Princess Diana, British royals typically used wet nurses. There may (or may not) be a nanny. But just one, and any royal siblings may share. (A far cry from the two to four nannies per child some American celebrities use.) However, sources tell the Daily Beast that Kate plans to be a full-time mom and doesn't want a nanny, an idea that initially shocked Prince Charles. She may, however, have a maternity nurse for the first few weeks. Speaking of royal siblings, Kate may be having twins. Medical experts say hyperemesis gravidarum, the severe form of morning sickness that caused Kate to be hospitalized, is linked to having twins, Radar reports. Kate wants her child to lead as "normal" a life as possible. Even so, that "normal" life will likely include a $2 million wardrobe in the first year alone, according to an expert who has dressed royals in other countries. But another expert notes that those clothes will probably be "sensible," and that royal babies are typically brought up with "few parties, not too many toys, and very dull meals."