Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge attend the 10th Annual ARK (Absolute Return for Kids) Gala Dinner at Kensington Palace on June 9, 2011 in London, England. Credit: Samir Hussein/

Hours after Kate Middleton and Prince William announced they will become first-time parents in 2013, their closest friends and family members offered congratulatory messages to the spouses of 19 months.

"It is wonderful news and I am thrilled for them both," William's uncle, Earl Spencer tells Us Weekly. (The 48-year-old author and journalist is the brother of the late Princess Diana, who was killed in an August 1997 car crash in Paris, France.)

Spencer himself has four children with ex-wife Victoria Lockwood (Lady Kitty Eleanor, Lady Eliza Victoria, Lady Amelia and Louis Frederick John, Viscount Althorp), three children with ex-wife Caroline Freud (The Hon. Edmund Charles and Lady Lara Caroline) and one child with wife Karen Gordon (Lady Charlotte Diana).

Prime Minister David Cameron congratulated William and his wife via Twitter. "I'm delighted by the news that the Duke and Duchess of Cambridge are expecting a baby," he wrote. "They will make wonderful parents."

The Archbishop of Canterbury, Dr. Rowan Williams, is also elated for the couple. "The whole nation will want to join in celebrating this wonderful news," said Williams, who presided over the couple's April 2011 nuptials. "We wish the Duchess the best of health and happiness in the months ahead."

Middleton, 30, has been hospitalized in Central London with Hyperemesis Gravidarum, an acute form of morning sickness that requires supplementary hydration and nutrients. "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," St. James's Palace tells Us.