ET's Lauren Zima caught up with Clarkson backstage at the 2018 iHeartRadio Music Festival at the T-Mobile Arena in Las Vegas on Saturday, and she opened up about watching Underwood -- who is pregnant with her and husband Mike Fisher's second child -- perform at the festival.
"Carrie's killing it, and she's pregnant! What?!" Clarkson exclaimed. "I hated singing pregnant."
As for Underwood's pregnancy journey, Clarkson said she was overjoyed at the "Cry Pretty" singer's emotional experience.
"I'm so happy for her!" the Voice coach shared. "I didn't know a lot of the situations that were happening behind the scenes."
Earlier this month, Underwood sat down for an interview with CBS Sunday Morning, where she revealed that she'd suffered three miscarriages in less than two years.
"I have a lot of friends who have gone through that several times as well," Clarkson shared.
Pregnant Carrie Underwood and Mike Fisher Overjoyed After 'Very Challenging Year' (Exclusive)
"It's a really cool thing [she did]. I was emailing her, because I was like, 'It's so important that you talk about it,'" she continued. "I know you don't have to, because it's a hard thing to do, but it makes women who feel inferior, or feel like something's wrong with them, it makes them feel comfortable, and [know] that people go through it."
"We have four, so I'm sure one of them will pop up every now and then," Clarkson joked, referring to the couple's 4-year-old daughter, River Rose, and 2-year-old son, Remington, as well as Blackstock's daughter Savannah and son Seth, from a previous marriage.