Fans flocked to the post to share notes of congratulations and well-wishes.
'That’s fab! Huge congrats ❤️,' one wrote, with another adding, 'Congratulations and God bless your baby girl.'
The couple are already parents to a four-year-old son.
Most recently, the pair celebrated their fifth wedding anniversary.
Mark wrote to social media, 'Happy Anniversary my love, 5 years! Words can’t describe how much I love you and what you mean to me, thank you for your love and support, thank you for the mother you are to Nicholas and soon to be daughter, we are all so incredibly lucky and grateful. 💙🙏🏼🌏😘 '