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William, Catherine celebrate fourth anniversary, await royal baby No. 2

Kate Middleton and Prince William mark fourth #royalwedding anniversary while waiting for #royalbaby No. 2

Well, we're back to Waity Katie.

With the arrival of Prince William and the former Kate Middleton's second child imminent, the couple marked their fourth wedding anniversary on Wednesday.

Catherine, Duchess of Cambridge, who waited several years before marrying her college sweetheart at Westminster Abbey in 2011, revealed earlier in her pregnancy that her due date was in "mid- to late April," and reports have claimed that the child's ETA was April 22 or 23.

The duo celebrated the four-year milestone, People reported, with a visit to the prince's grandma's house: Buckingham Palace.

If the spare heir -- who would replace William's younger brother Prince Harry as fourth in line to the British throne -- is indeed past his or her due date, experts said, it could be up to two weeks before doctors would induce labor.

Jane Munro, midwife and advisor at the Royal College of Midwives, told People that the number of days may vary, but guessed that doctors would wait until around 12 days past her due date before inducing delivery. 

"The baby may be getting big, and the placenta functions less well after that time," Munro said.

Two weeks past due is said to be the cut-off for waiting for natural labor, the mag reported.

Royal well-wishers and monarchists have been camped out in chilly weather outside St. Mary's Hospital for days, anticipating the moment Wills and Kate will head to the Lindo Wing to welcome their little prince or princess. That's also where the royals welcomed their firstborn son, Prince George, on July 22, 2013.

Still no word on whether they're having a boy or a girl, but oddsmakers are putting their money on a little princess.

On Tuesday, the couple surprised waiting spectators with breakfast -- coffee, croissants and pastries -- and its packaging further fueled the it's-a-girl speculation. Why? The white pastry boxes were tied up with pink ribbons. That has to mean something, right? The betting folk think so.

"The Duke and Duchess have seen the news reports about these people camping out and they just wanted to let them know they were thinking of them," a palace spokesman told the Daily Mail.

Also on Tuesday, Kate was spotted driving her Range Rover to Buckingham Palace with up to five bodyguards in tow. The heavily pregnant duchess was reportedly taking 21-month-old George for a swim at the queen's private pool, the Mail said. (Going swimming at great-grandma's house, brb!)

The couple is currently staying at their Kensington Palace home while they await their second child's arrival. Prince William started his paternity leave ahead of schedule earlier this month after he completed a phase of air ambulance training, E! News reported.

"This is due to a combination of the Duke performing well in the various modules and having very good flying conditions that allowed flight training and assessment to progress more quickly than had been planned," a Kensington Palace representative told the publication.

The prince was training at a base in Norwich, a little more than 100 miles northeast of London. It's said he will remain on unpaid paternity leave until June 1.

Follow me on Twitter @NardineSaad.

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