Mia is currently 20th in line to the throne and while her position in the line of succession will remain the same if Zara has a boy, there is another reason why this will soon change.
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This means everyone below the siblings in the current line of succession will move further away from the throne, including the Duke of York, Princess Beatrice, Princess Eugenie and her baby son, August Brooksbank, the Earl and Countess of Wessex and their children, as well as the Princess Royal, Peter Phillips and his children, Zara, Mia and Lena.
When baby Sussex arrives, Mia's position in the line of succession will move out of the top 20 to 21st in line, while her younger sister Lena will be 22nd and their new baby brother or sister in 23rd.
When Mia was born in January 2014, she was 16th in line to the throne. She and Lena were not given royal titles at birth, just like the decision Princess Anne made with Peter and Zara.
Mike with daughters Mia and Lena in 2019
Former rugby star Mike recently spoke about why he and wife Zara won't be finding out the sex of their third child.
In an interview with The Times, he said: "It was always an interesting question whether we'd go for a third and I think both of us wanted a boy, so hopefully we'll get one.
"We're not finding out - I think it's better that way. At the end of the day, you're just happy if it's got ten fingers and ten toes and it's healthy.
"And Mia's such a Daddy's girl, it's been brilliant. I've loved having girls."
Mike and Zara live with their children on Princess Anne's Gatcombe Park estate in Gloucestershire.
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