Questions over the child's paternity were rampant on social media earlier in the week, and Fox responded with a tongue-in-cheek hashtag of her own. On Tuesday, the Teenage Mutant Ninja Turtles: Out of the Shadows star posted a collage of photos, including images of Shia LaBeouf, Jake Johnson and Arnett, along with the caption "#NotTheFather."
Arnett responded to the post at CinemaCon in Las Vegas on Tuesday afternoon, joking to ET that he's shocked he's not the parent.
"She said I'm not the father?" Arnett quipped. "That's a big surprise to me."
Fox's baby bump was on display when she walked the red carpet with her TMNT costar on Monday, after reports of her pregnancy had surfaced earlier in the day.
"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 continued with a laugh. "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."
Despite questions of who the father might be, a source confirmed to PEOPLE that Brian Austin Green is "of course" the father and that the pair "never stopped loving each other and they will never stop being devoted parents to their boys.
However, the actress filed for divorce in August after five years of marriage and 11 years together. According to legal filings, the divorce is still pending. Reps for both stars have not commented.
The pregnancy came as a surprise for the couple, who are "very excited" and are "continuing to work on their marriage and are living together again," another source said.