"Bey and Kate talked about their kids," an eyewitness told E! News. "They asked each other, 'How old is yours?' They basically bonded over motherhood."
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The Duke and Duchess of Cambridge met with Beyonce and Jay Z in New York
Beyoncé, 33, has a daughter Blue Ivy, who will be three in January, while the pregnant Duchess, 32, is mummy to 16-month-old Prince George, who is due to become a big brother in April.
They also appeared to be in the holiday spirit – "Both also wished each other a Merry Christmas," the insider revealed, adding that although the conversation was brief, it was "a very warm and friendly one".
William and Kate's meeting with the music power couple was one of the highlights of their three-day whistlestop trip to New York.
Beyonce and the Duchess bonded over motherhood
Babies were also the main topic of conversation when the royals met with Hillary Clinton, Chelsea Clinton and her husband Marc Mezvinsky during their stay in the Big Apple.
William and Kate had the opportunity to congratulate the former First Daughter on the birth of her daughter Charlotte five months ago – and she returned the favour, enquiring how the Duchess' second pregnancy was going.
During their stay in the Big Apple, the royals also visited the 9/11 memorial, and William hopped on a commercial flight to Washington to meet with President Barack Obama. Their tour came to a close on Tuesday evening, with a glamorous appearance at a fundraiser for St Andrews university – where William and Kate first met and fell in love.