The couple are already parents to August Brooksbank, who will turn two on 9 February. Check out little August's adorable reaction at the Platinum Jubilee last year in the video below...
WATCH: August Brooksbank joins parents Eugenie and Jack at the Platinum Jubilee
Another new addition to the royal family will mean some changes to the line of succession when Eugenie's baby girl or boy is born.
Eugenie's aunt, the Princess Royal, will become 17th in line to the throne. At the time of her birth in 1950, she was only third in the line of succession.
It also means changes for Princess Anne's children and grandchildren too. Peter Phillips and his daughters, Savannah and Isla, will move to 18th, 19th and 20th.
Meanwhile, Anne's daughter, Zara Tindall, will move out of the top 20 to 21st in line, while her children, Mia, Lena and Lucas, will drop down to the 22nd, 23rd and 24th positions.
Mia, Lena and Lucas will move further down the line of succession
Charlotte became the first female royal to benefit from The Succession to the Crown Act (2013), which states that girls will not be overtaken by younger brothers.
When Mia and Lena's brother, Lucas, was born in March 2021, he did not overtake his older sisters in the line of succession.
Princess Eugenie shared her happy news on Instagram
Meanwhile, like big brother, August, Eugenie's second child will not have a royal title and will be known as Miss or Master.
Eugenie's mother, Sarah, Duchess of York, was one of the first to react to her daughter's special news.
Taking to her personal Instagram account, the Duchess posted an adorable shot of her eldest grandchild, August, wearing a yellow raincoat as he jumped through puddles.
"You will be sharing puddles, Augie ! Superb news, Granny heaven…. So deeply grateful," she captioned the post.
Listen: The Harry and Meghan A Right Royal Podcast special
Make sure you never miss a ROYAL story! Sign up to The Royal Explainer newsletter to receive your weekly dose of royal features and other exclusive content straight to your inbox.