The 29-year-old attended CinemaCon, the official convention for The National Association of Theatre Owners, on Monday 11 April wearing a tight-fitting black dress that appeared to show a "baby bump" when she turned to the side.
Chris Pizzello/Invision/AP
Fox is already mum to Noah, four, and Bodhi, two, with ex-partner Brian August Green.
The couple announced their split in August 2015, but their divorce is still pending.
According to PEOPLE, Fox and Green have been spotted together several times in the past couple of months running errands without their children.
Fox is yet to confirm the news she is pregnant.