The question of the baby's paternity reached fever pitch on Tuesday when Fox jokingly tweeted a three-way split image of former co-stars Shia LaBeouf, Jake Johnson and Arnett, with the caption, "#NotTheFather."
"She said I'm not the father?" Arnett responded, feigning shock when he spoke to ET’s Cameron Mathison at CinemaCon in Las Vegas on Tuesday afternoon. "That's a big surprise to me."
Fox showed off her barely-there baby bump while walking the red carpet at CinemaCon on Monday alongside her Teenage Mutant Ninja Turtles co-star, after news broke earlier that day that she was expecting.
Fox already shares two sons -- 3-year-old Noah and 2-year-old Bodhi -- with her estranged husband, Brian Austin Green. Though their divorce is not finalized, Fox filed papers to end their marriage in August 2015 citing "irreconcilable differences."
"There are like 50 jokes that come to mind but we live in social media era and I'm not going to say 'em you know what I mean?" Arnett laughed. "My publicist just shook his head at me. He knows me really well. There's so many things I would say if [these cameras] weren't rolling."
While Green and Fox split months ago, the two have frequently been spotted spending time together with their children, going on outings in Los Angeles. It has also been noted that Green did not appear in the #NotTheFather Instagram pic Fox posted.
See Fox's tiny baby bump on full display in the video below.