went through "pain, loss and disappointment" during her fertility struggle.
The 43-year-old actress - who welcomed Faith Margaret, her second child with country music singer Keith Urban, into the world on December 28 via a gestational carrier - admitted she suffered agonising times when trying to conceive.
She explained: "Anyone that's been in the place of wanting another child or wanting a child knows the disappointment, the pain
and the loss that you go through trying and struggling with fertility.
"Fertility is such a big thing, and it's not something I've ever run away from talking about."
The 'Rabbit Hole' star - who also has a two-year-old daughter Sunday Rose with Keith, as well as adoptive children Isabella, 18, and Connor, 15, with ex-husband Tom Cruise
- explained she and her country singer spouse were in a position where while they were "desperate" for another child, but she couldn't get pregnant.
Nicole added to Australian TV show '60 Minutes': "We were in a place of desperately wanting another child. The opportunity arose for us, and I couldn't get pregnant.
"Children are children - you'll die for your children. And when you feel that as parent - that's the unconditional love."