Britain's Prince William stand next to his wife Kate, Duchess of Cambridge

Kate Middleton and Prince William leaving hospital on Thursday. Photo: AP

Days after being discharged from hospital, Kate Middleton has turned down an invitation to attend a movie premiere to “rest privately”.

Diagnosed with hyperemesis gravidarum, a pregnancy condition likened to acute morning sickness, the Duchess spent three nights in King Edward VII hospital before returning to Nottingham Cottage with Prince William on Thursday.

She was scheduled to return to her royal duties this week, previously indicating she would attend a red carpet movie event, but will now continue to recover at home.

"The Duchess of Cambridge will not attend The Hobbit Premiere tomorrow evening and will continue to rest privately," a St. James Palace spokesperson told Us Weekly. Prince William, however, "will attend as planned."

Prince William spoke publically about his wife’s illness yesterday, saying, “I don't know why they call it morning sickness, they should call it all day and all night sickness. [Recovery is] a long old process but she is getting there.

“She feels like it is going to go on forever."

Hyperemesis gravidarum can affect sufferers for months, and even last the entire pregnancy.

Most women with the condition will be admitted to hospital, where they'll be rehydrated through an IV and given sustenance intravenously. The treatment also includes anti-nausea medication.

Early speculation

A friend of the couple had said that the Duke and Duchess were forced to announce the pregnancy earlier than planned, and that they had wanted to break the news to family members over Christmas.

Jessica Hay, who was the first person to speak to the media about the baby rumours, has said that the baby is due in mid-July.

Hay also told New Idea magazine that the couple first learnt of the pregnancy after doing a test at home.   

She said doing the test was “most nerve-wracking moment” of Middleton’s life, and that the Duke and Duchess “celebrated with a home-cooked shepherd’s pie and an early night.”

The UK’s Daily Mail has reported, even at this very early stage, that the baby is believed to be a girl. The paper pointed to the idea that "hyperemesis gravidarum is more commonly found among women expecting girls", a claim that appears to be based on anecdotal evidence rather than evidence-based research. 

The paper also used the Prince's occupation as an RAF search-and-rescue pilot as a gender predictor, stating that "an American study found pilots had an 80 per cent chance of having girl babies". However, other experts have queried results of the 1987 study in the past.

Regardless of the rumour mill, the royal family is said to be thrilled with the pregnancy.  

"It's a very nice thought to become a grandfather in my old age, if I can say so," Prince Charles said at an event in London after the news broke.

For more pregnancy news, including details on Holly Madison's hospital stay, check out our gallery.