Los Angeles, California - Model and fashion designer Nicky Hilton and her husband James Rothschild have welcomed their third child together.

"Welcome to the world, sweet boy," Hilton wrote on Instagram on Tuesday, accompanied by a black and white photo of the couple showing off their baby bump.

"We are officially a party of 5!" she wrote.

She did not disclose the new bundle of joy's name or birthdate.

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Hilton and Rothschild are also parents to daughters Lily Grace Victoria (5) and Theodora "Teddy" Marilyn (4).

"Mom, dad and big sisters could not be more smitten," she added.

Nicky Hilton is the younger sister of Paris Hilton and the great-granddaughter of hotel entrepreneur Conrad Hilton of the Hilton hotel chain.

She married British banker Rothschild in 2015, and the wedding was celebrated at London's Kensington Palace.

The 38-year-old was previously married to financial advisor Todd Meister in Las Vegas in 2004. The union was annulled after three months.

While Nicky has three children, her star sister Paris has yet to have kids of her own. Paris has been vocal about trying for children with her new hubby, and while there have been pregnancy rumors, she's denied the false claims.