The How I Met Your Father and Lizzie McGuire star revealed that her family is growing as she shared an incredibly cute Instagram post that initially left followers guessing.
The first slide shows a Christmas card photo of her, her son, her two daughters and her husband Matthew Koma, each sitting in their pajamas.
Looking closer, the frame around the photograph bears the words: 'Enough for a silent night' – while Hilary is seen holding a baby bump and looking shocked.
The next slide showed several adorable photos of their family, along with a small message: “Fasten your seatbelts, buttercups! We're adding one more to this crazy group!'
And in the caption, the mother-to-be wrote: 'Surprise surprise!'
Fans flocked to the comments to congratulate the family, with user Onekingslane saying 'this is what dreams are made of! Congratulations to your growing family!
User Teresa King said 'oh how cute, congratulations' while Post.Partyy said 'yayyy, congratulations, beauty!'
And only1dharti shared her own story, writing: 'Congratulations!!! I'm one of four brothers… I wish you all the best of luck 😂 but also know that it's so fun to have a group of friends!
Hilary is already mum to Luca, 11, from her marriage to Mike Comrie, and shares two daughters with her husband Matthew.
The couple welcomed their first daughter, Banks, in 2019, with Mae two years later.
It's a joyous end to the year for Hilary, who suffered huge disappointment when the How I Met Your Mother spin-off, How I Met Your Father, was canceled after two seasons.
Hilary played the main character Sophie in the sitcom, which also starred Kim Catrall as her older self.
The cancellation meant that viewers would never find out who the father of Sophie's child is, which in itself is disappointing.
However, Hilary had quick words for those happy about the cancellation and savaged an Instagram commenter who called the show 'cringe'.
The star, who has 26 million followers on Instagram, simply wrote: 'You have 63 followers.'
Hilary previously said Metro.co.uk what she was nervous about doing a sequel to such a beloved comedy.
'When I got the call about the role, I thought, “What's the title? No no no no!” Very intimidating. A lot of pressure because HIMYM was so turned on, so loved and the cast was amazing,” she recalled.
“But I was talking to the producers and they said they weren't trying to stage anything and that it's a really brilliant way to tell a story.”
Do you have a story?
If you've got a celebrity story, video or pictures get in touch with the Metro.co.uk entertainment team by emailing us email@example.com, calling 020 3615 2145 or visiting us Send stuff – we love hearing from you.
MORE: Hilary Duff shares powerful message after death of man who changed Lizzie McGuire's life
MORE: Hilary Duff Has a Sassy Response to the Person Who Ridiculed How I Met Your Father's Cancellation