Fergie - who shares two daughters with Prince Andrew - added she was proud of both her daughters, following Princess Eugenie's wedding last year at Windsor Castle to Jack Brooksbank.
She said: "I'm really proud of them both."
"And for mums out there, oh my God. I've never understood what it is to have tears of joy because I looked at mothers at weddings, I'm going 'yeah, yeah, yeah, it's good news'.
"No, you really feel it because they're your babies."
When news of Princess Beatrice's engagement first broke, Fergie penned a heartfelt message for social media.
The Duchess shared several engagement photos of the pair to Instagram, with the following caption:
"I know what a mother feels so I have tears of joy. I am so proud of this sensational news. Andrew and I are just the luckiest people ever to have two great sons in law. @princesseugenie took the colour photographs as my girls are so close. Beatrice and Edo wanted her to capture this special moment."
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The royal wedding is set to take place in 2020.
The Royal Family Instagram account shared a post featuring photos of Beatrice and her fiance, along with the following statement:
"The Duke and Duchess of York are delighted to announce the engagement of Princess Beatrice to Mr Edoardo Mapelli Mozzi.
Her Royal Highness and Mr Mapelli Mozzi became engaged in Italy earlier this month. The wedding will take place in 2020. This photograph was taken by Princess Eugenie in Italy.
Princess Beatrice and Mr. Edoardo Mapelli Mozzi said; 'We are extremely happy to be able to share the news of our recent engagement. We are both so excited to be embarking on this life adventure together and can’t wait to be married.'
