"I don't even know what to say about these things," she said on Thursday's "The Ellen DeGeneres Show" when asked about the rumors. "I feel 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."
Biel, 34, said she even joked about the rumors with her husband. "I was like, 'Are we? No? OK, cool,' she recalled, before quipping, "He usually knows more about everything than I do."
For the time being, Biel and Timberlake, 35, have their hands full with their son, Silas, whom they welcomed last April.
The actress said her son is a "very musical" baby.
"Silas loves his music, so I want him to hear it a lot," she added. "We listen to it in the car a lot."
Silas, who will turn 1 next month, already has a favorite singer.
"He is really, really in love with Erykah Badu," Biel revealed. "He loves her music. He beats to her music. He loves that. He's gotten, just recently, these drum sets. He likes to drum, and he likes to play his piano and he likes to drum with his head...which I just think that's genius, if you ask me."