"I'm pregnant!!! Brandon and I are so excited! Best early Christmas present ever :)," the first-season American Idol winner announced on Twitter Tuesday.
Clarkson, 31, wed new husband Brandon Blackstock last month. The couple got engaged on December 15 of last year. Blackstock already has two children, ages 6 and 12, from his prior marriage to Melissa Blackstock. .
Rumors had circulated Clarkson was pregnant around the time she got married because she had suddenly ditched her plans for a big fall wedding and ended up quickly having an intimate ceremony in Tennessee.
While Clarkson initially shot down such speculation, she had hinted that may have changed during an appearance on The Tonight Show with Jay Leno last week.
"Rumors are you want to have a baby," Leno asked.
"Now, like now. Practice makes perfect!" Clarkson said with a smile. "We're trying, trying, trying!"
"Are you pregnant right now?" Leno asked.
"What are you saying?!" Clarkson said with a big laugh, dodging the actual question.
"I'm asking you..." Leno began.
"What are you saying?!" Clarkson interjected.
"I always get these stars that come on [my show] and say, 'Oh no, I'm thinking of having a baby.' And then the next week, they go on [The Ellen DeGeneres Show] and announce it there. I'm going to ask you right now. Are you pregnant? Remember, you are under oath!" Leno asked, getting straight to the point.
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"You're not -- what?!" an overwhelmed Clarkson said in surprise. "We are not announcing anything right now. We're not, no."
Blackstock is the son of Narvel Blackstock, Clarkson's manager, and stepson of country singer Reba McEntire, who became friends with Clarkson after the women performed a duet on American Idol in 2002.
Clarkson signed with Narvel Blackstock's Starstruck Entertainment management company in 2007 following a feud with recording industry titan Clive Davis over the musical content of "My December," her third album.