Is Kate Middleton pregnant with twins? After the royal family announced her pregnancy, the Duchess of Cambridge remains in hospital with acute morning sickness known as hyperemesis gravidarum. The rare condition results in weight loss and is often associated with having twins.

Women diagnosed with hyperemesis gravidarum typically lose 5 per cent of their pre-pregnancy weight, says Dr Ashley Roman, a professor at the New York University Langone Medical Center. "It's traditionally thought that nausea and vomiting is a sign of a healthy pregnancy." Dr Nancy Cossler at University Hospitals in Ohio said the Duchess and her family should not be worried, "The biggest problem with this is how it interferes with your life.Constantly feeling sick and puking is difficult." Cossler explains that the condition is thought to be caused by higher levels of the pregnancy hormone hCG, which can often mean that the woman is carrying more than one foetus. Though there's very little data on twins and hyperemesis gravidarum, one study shows that women carrying twins had a 7.5 per cent higher risk of experiencing the acute morning sickness - a considerable increase in the grand scheme of things. According to The Independent newspaper, there is evidence that mothers-to-be who suffer from the condition are three times more likely to have a multiple birth than other woman. 

The severe type of morning sickness is usually diagnosed about nine weeks into the pregnancy, however, in rare cases it can last the whole pregnancy. "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," read a statement from St. James Palace. Middleton's husband Prince William is currently by his wife's side at a private hospital in central London. 

The expectant parents have received congratulations from world leaders including President Obama, whose press secretary said, "On behalf of everyone here in the White House, beginning with the President and the 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." Prime Minster David Cameron said, "It's absolutely wonderful news and I'm delighted for them. I'm sure they will make absolutely brilliant parents, and I'm sure everyone around the country will be celebrating with them tonight."

Bookmakers are already offering odds on Kate and Will's chosen baby-name, with George and Victoria the current favourites at 10/1. Ann, Charles, Edward and Elizabeth are around the 12/1 mark, while ambitious punters can have Hashtag at 500/1.