Kate Middleton officially became a part of the royal family when she married Prince William at Westminster Abbey on Apr. 29, 2011. Though she was given a warm welcome by everyone from Queen Elizabeth II to Prince Charles, the Duchess of Cambridge has become especially tight with her brother-in-law, Prince Harry, since announcing her pregnancy in December 2012.

PHOTOS: All of Kate Middleton's maternity looks

"The baby has brought them closer," a palace insider reveals in Us Weekly's Raising a Royal Baby. "He told her not to be afraid to call, no matter the time of day." As it happens, Harry's flat is a mere 150 feet away from Middleton's two-bedroom Nottingham Cottage on the Kensington Palace grounds.

PHOTOS: Kate Middleton's best hairstyles

Middleton, 31, was due to give birth on Thursday, July 11. Harry, 28, returned to the United Kingdom just one day earlier after spending three days training with the Walking With The Wounded South Pole team in Iceland.

PHOTOS: Prince Harry's tour of duty in Afghanistan

Harry first visited the country in March, as he is training to raise funds for the charity in November. (Walking With the Wounded focuses on funding rehabilitation and education for soldiers that have been wounded in combat. Harry and his team have been training for the challenge on the Langjokull glacier.)

PHOTOS: How Kate Middleton is getting ready to be a mom

"We're both honored and impressed that he chose to fly with us," a spokesperson for WOW Air, a low-cost Icelandic airline, said in a statement. "It goes to show that despite offering cheap flights to Iceland, WOW Air is fit for a king!"

This article originally appeared on Usmagazine.com: Kate Middleton and Prince Harry "Became Closer" During Her Pregnancy