The 34-year-old actress, who shares 11-month-old son Silas with the singer, addressed the rumors on Thursday's episode of The Ellen DeGeneres Show.
"I don't even know what to say about these things," Biel said. "I was like, 'Man, I thought I was doing good with my yoga practice. I thought I was in the gym,' but no, I have a gut apparently."
"I heard that I called Justin. I was like, 'Are we?' He was like, 'No,'" she joked. "He usually knows more than I do about everything."
Biel and Timberlake married in 2012 and welcomed Silas in April. The actress admitted to often playing Timberlake songs for her son, who has inherited his 35-year-old dad's love for music.
"He is really, really in love with Erykah Badu," she said of Silas. "He's gotten, just recently, these drum sets. He likes to drum, he likes piano and he likes to drum with his head. Which I just think is genius, if you ask me."
Biel recently told Extra the best part of motherhood is when her son's face lights up when she comes in the room. The actress will next star in The Devil and the Deep Blue Sea, and opened a restaurant, Au Fudge, in Los Angeles this month.