Hilary Duff reveals she's pregnant with her fourth child: 'So much for silent nights!'

Hilary Duff has announced she's pregnant and expecting her fourth child in a heartwarming new post.

The actress, 36, revealed the happy news on Instagram on Wednesday, sharing a snap of her family's holiday card.

"So much for silent nights... Buckle up buttercups, we're adding one more to this crazy bunch!" the card read.

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It continued: "The Duff, Bair, Comrie crew."

In one image on the card, taken by photographer Jenna Elliott, Duff cradles her baby bump while wearing a pair of pink silk pyjamas while her family pose around her.

"Surprise surprise," Duff captioned the post, which was flooded with well wishes from friends and fans.

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Duff shares daughters Mae, two, and Banks, four, with musician husband Matthew Koma, 36, and shares son Luca, 11, with her ex-husband and former ice hockey star Mike Comrie.

"Ahh congrats you guys!" actress Lucy Hale commented underneath.

Stars Dakota Fanning and Ashley Benson also sent well wishes.

Former Lizzie Maguire actress Duff recently spoke about motherhood to People and said that she "loves this mayhem".

"I'm so in love with all my kids," she told the magazine.

She also told Shape: "I love my loud household with mayhem and the craziness and all the love and all the dirt."

"I love [parenting] with my husband... I love the pressure of being like, 'How the f--k are we going to make it through this day?' And making it through the day and being like, 'I don't know how everyone is tucked in and fed and happy after all the things we've had to do today.'"