Ayda Field Williams shared the news on her Instagram account on Friday 7 September, explaining why they’d kept the news on the down low.
Posting a photo of her family’s hands - including their two children Theodora, five, and Charlton, three, the mum wrote: “I spy with my little eye an extra little hand.”
She continued: “We have been keeping a very special secret. We are delighted to share with you that we have had a baby girl… welcome to the world Colette (Coco) Josephine Williams. It has been a very long and difficult path to get here.”
The mum-of-three continued by telling her fans she and Robbie welcomed a child via a surrogate.
“Family comes in all forms, and this little lady, who is biologically ours, was carried by an incredible surrogate mother, to whom we will be eternally grateful,” she wrote.
“We are over the moon to have this beautiful baby girl in our lives and so blessed that we live in a world which makes this possible. As with Teddy and Charlie, we ask that you respect Coco’s privacy and allow us to grow into our new team of five.”
The couple, who are both currently judges on ‘The X Factor’, were showered with messages of congratulations. “Omg amazing news this is wonderful,” one person wrote.
Another commented: “Inspiring that you have shared your surrogacy story with us. Thank you.”
Other celebrities who have had children through surrogacy include Tom Daley, Nicole Kidman, Lucy Liu And Sarah Jessica Parker.