The pregnant Duchess of Cambridge was attending Queen Elizabeth II's Diplomatic Reception at Buckingham Palace, and was photographed wearing the Cambridge Lover's Knot tiara.
The spectacular piece was loved by Diana.
Kate has previously worn this tiara to the same event, three years in a row! She was snapped wearing the same piece at the 2015 and 2016 Diplomatic Receptions.
The tiara was known to be gifted to Princess Diana by Queen Elizabeth II in 1981 on her wedding day to Prince Charles. When Middleton wore the stunning headpiece in 2015, it was the first time it had been worn since Diana's passing in 1997.
The tiara has an even greater history though, reportedly being commissioned by Queen Mary in 1913. When Mary died in 1953, the tiara was gifted to Queen Elizabeth.
Also attending the event was Camilla Duchess of Cornwall and Prince Charles. Camilla was snapped wearing the late Queen Mother's Greville tiara, which was named after a socialite who gifted it to her in 1921. After the Queen Mother passed away in 2002, the tiara went back to the Queen who regularly loans it out to Camilla.