Earlier this week, several outlets published a photo of Hannigan out with her husband Alexis Denisof sporting what was described as a 'baby bump'.
However, the former Buffy The Vampire Slayer actress has insisted that it was simply a bad photograph and that she isn't pregnant.
"No, I'm not pregnant!" Hannigan tweeted earlier today. "I just ate too much carnival food, that time of the month, hurt my back & couldn't suck my gut in, & need to do cardio!"
"And I'm never gonna wear that shirt again!" she later added.
Hannigan and Denisof welcomed their first child, a daughter called Satyana, back in 2009.
The actress recently finished filming scenes for American Reunion, the fourth theatrical entry in the American Pie series.
Watch the brand new teaser trailer for American Reunion below: