Megan Fox is pregnant with her third child and the father is her estranged husband Brian Austin Green.

Fox, 29, debuted her burgeoning bump on Monday while promoting her new film, Teenage Mutant Ninja Turtles: Out of the Shadows, immediately prompting speculation about who was the father.

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Fox and Green filed for divorce in August 2015 after five years of marriage but have remained friendly for the sake of their two sons, Noah, 3, and Bodhi, 2.

Fox responded to rumours about her child’s paternity with a tongue-in-cheek Instagram post ruling out possible fathers.

She captioned the collage of fellow actors Shia LaBeouf, Will Arnett and Jake Johnson “#notthefather”.

Neither Fox nor Green have made an official comment on the pregnancy.

However, a source close to the couple confirmed to People magazine that Green is the baby’s father despite the couple’s separation.

“Of course Brian is the dad!” the source said, “they never stopped loving each other and they will never stop being devoted parents to their boys.”

The source said the actors are living together again and will continue to work on their marriage.