Satirical website Celebrity Jihad, which regularly posts false images of celebrities, put up an edited photo showing the Transformers actress naked. This prompted Fox's lawyers to issue a cease and desist letter to the site.
The letter claims that the photograph is fake and "must be taken down ASAP", or her lawyers will sue.
A representative for Celebrity Jihad told TMZ: "While we appreciate Megan Fox's concern for her image, we find it hard to believe that a woman who spent two Transformers movies bent over with her breasts pressed together could have her reputation damaged by a blatantly satirical website."
The 26-year-old, married to Brian Austin Green, gave birth to her first child Noah Shannon on September 27. The actress took to Facebook to announce the news on October 17.
A disclaimer on Celebrity Jihad's site reads: "CelebJihad.com is a satirical website containing published rumours, speculation, assumptions, opinions, fiction as well as factual information. Information on this site may or may not be true and is not meant to be taken as fact. CelebJihad.com makes no warranty as to the validity of any claims."