X Factor judge Field, 39, said it had been a "very long and difficult path to get here".
Colette (Coco) Josephine Williams, who is "biologically ours" and carried by "an incredible surrogate mother", is the newest addition to the family and will be sister to Teddy and Charlie.
Field posted a black-and-white image on Instagram with the baby's hand alongside those of her siblings and parents.
"I spy with my little eye an extra little hand," she wrote.
"It has been a very long and difficult path to get here, which is why we have kept it on the down low."
She added: "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.
"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 5! #TeamWilliams".
Singer Williams, 44, and Field are currently appearing on screen together as X Factor judges.
Story by Press Association