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.
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.
Fox is yet to confirm the news she is pregnant.
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