The 47-year-old star of the new Paddington film has two children with her husband Keith Urban, 47, - Sunday Rose, six, and Faith Margaret, three, who was born via a surrogate mother.
She also adopted two children while with her ex-husband Tom Cruise - Isabella, 21, and Connor, 19.
But Nicole doesn't yet feel that her family is complete.
The personal revelation came during an interview with a pregnant journalist for Elle magazine's January 2015 issue.
The interview was meant to focus on Nicole's career, but when the interviewer stood up and Nicole spotted her baby bump, she couldn't resist talking babies.
"Oh my gosh, you are so lucky," she said. "Is this your first?"
When the interviewer told Nicole that the baby will be her first - and "quite possibly my only - at 38, I'm of 'advanced maternal age'." Nicole is quick to reassure her that older mothers can still conceive.
"I had my last one at 43," she responds. "My grandmother had her last at 49. Every month, I'm like, 'I hope I'm pregnant!'"
In the new Paddington film Nicole plays a villainous taxidermist called Millicent, who wants to see Paddington Bear stuffed. See a behind-the-scenes clip from the new movie below: