Keith Urban started to wonder if he was pregnant when he broke down in tears at Kelly Clarkson's American Idol comeback.

Kelly struggled to control her emotions as she performed Piece By Piece on the show on Thursday night (25Feb16), also reducing Keith into a sobbing mess.

After the show, Keith admitted it was the intensity of Kelly's performance that had affected him the most, although his hormones also went crazy.

"I was starting to wonder if maybe I was (pregnant) too," Keith joked to Entertainment Tonight.

"The humanity in that moment was just (incredible). You never see that, let alone on TV, so I was so grateful to be there to witness it."

Kelly is nine months pregnant with her second child, and penned her new song about being abandoned by her father when she was a child. Listening to the lyrics of the song, Keith realised the depth of its meaning.

"The song, it's insane," he added. "I figure you're either crying at this performance or you need to check your pulse. It's an amazing song."

An emotional Kelly, who won the first season of American Idol, told host Ryan Seacrest afterwards: "I actually wrote it pregnant with my little girl and now I'm pregnant with my little boy. So if I don't make it through and I start crying, just laugh. It'll make me feel better."

Keith is currently incredibly busy, with his commitments on American Idol and his ripCord world tour. He also has two daughters with wife Nicole Kidman, and spoke recently about how the actress helped him get over his demons by pushing him to go to rehab shortly after their 2006 wedding.

"(She's) the one I’ve been looking for and the one I’ve been preparing for, or life has been preparing me for, anyway," he told The Boot. "We got married, and I hadn’t taken care of my problems, and so my demons just came and kicked my a*s and her a*s and blew our world up. But this woman rallied the troops...

“In so many ways, it was the most powerful act of love in reaching into the fire and pulling me out, because she believed in me, and she saw something in me in such a way that she risked everything, everything, to trust me and pull me out of the fire and let me then do the right thing and move into the light."