Will Arnett plans to buy Megan Fox a "birthing tub".

Will Arnett

Will Arnett

The 46-year-old actor thinks his "hippy-dippy" 'Teenage Mutant Ninja Turtles: Out of the Darkness' co-star - who is expecting her third child with estranged husband Brian Austin Green - would love the gift so she can get in and "chant" while giving birth.

He said: "She is hippy-dippy. I should buy her Birkenstocks or something or a birthing tub.

"That's what I'm going to get her. It's like a whole tub and they get in it and everyone's in it at the same time...Chanting and all the fluid is happening at once."

A day after it was revealed she was pregnant to a mystery father, Megan - who also has sons Noah, three, and Bodhi, two, with Brian - posted an Instagram collage featuring Will, her 'Transformers' co-star Shia LaBeouf and fellow 'New Girl' cast member Jake Johnson and added the caption "#NotTheFather".

And Will - who has kids Archie, seven, and Abel, five, with ex-wife Amy Poehler - thought the picture was funny, though he had several jokes about the possible paternity riddle, but didn't dare repeat them in public.

He told E! News: "It was pretty humorous.

"I got asked about it about a million times and I had so many great answers that I'm not allowed to say.

"There's no way because it would instantly be retweeted a hundred million times. It's super rude and uncool - but it was great."

Meanwhile, Will can't wait for his sons to see the new 'TMNT' movie.

He said: "It's super fun [and] by far the best 'Ninja Turtles' movie ever made. I'm happy I get to make it because my kids will see it. It's not something they have to wait until they're like in recovery to watch and for me that's like a plus. I'm psyched. "

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