Keith Urban has now revealed their battle to provide a sibling for daughter Sunday Rose inspired the newborn's name.
He tells Access Hollywood, "Just the way she came into our world - we needed to have every bit of faith we could muster. It was completely out of our control. We just kept saying, 'We gotta have faith, we gotta have faith. She's going to be fine.' And we just thought, 'What a perfect name for her'."
The baby was named Margaret after Nicole Kidman's paternal grandmother, who welcomed a child at the age of 49.
The actress adds, "Margaret is the name of my dad's mother, my grandmother. She had her last baby when she was 49, so, she's my inspiration."
Nicole Kidman also reveals little Sunday Rose is overjoyed to have a sister, adding: "She's so into (Faith) and she's crazy about her. We really wanted Sunday to have a sibling, you know, so, so now we've got Sunday and Faith Margaret... we've got our hands full! We're really happy."