Mum-to-be Eugenie also shared her own personal announcement on her social channels.
In the two snaps, she shared a sweet photo of herself and husband Jack, plus a close-up of their hands holding a pair of adorable fluffy baby shoes.
"Jack and I are so excited for early 2021...." she captioned the snap.
New Idea reported earlier this month that the royal was four months pregnant with her first child.
Royal author Phil Dampier said: "It will be tremendous news for the royals if Eugenie is pregnant.
"The Queen and Prince Philip will be delighted to have yet anohter great grandchild and it would be the perfect present next year for Philip's 100th birthday.
"It's been a rotten year for Eugenie what with the Epstein scandal engulfing her father Prince Andrew so this is the best tonic for the whole family.
"For once, it will take the headlines away from Andrew and his association with Jeffrey Epstein."
Baby Brooksbank will be not only the couple's first child, but Prince Andrew and Sarah Ferguson's first grandchild.
The royal baby will also be the Queen and Prince Philip's ninth great-grandchild.
Rumours of Eugenie's pregnancy have been abuzz ever since the couple tied the knot in October 2018.
The princess and businessman wed in a lavish ceremony at St George's Chapel at Windsor Castle surrounded by friends and family.
Congratulations to the parents-to-be!