"NO, I'm not pregnant!" Hannigan clarified via Twitter Sunday. "I just ate too much carnival food, that time of the month, hurt my back & couldn't suck my gut in, and need to do cardio!"
UsMagazine.com said Hannigan's message addressed pregnancy rumors sparked by a photo showing her clad in a loose-fitting top while attending the Malibu Chili Cook Off and carnival.
"And I'm NEVER gonna wear that shirt again!!!"
the actress said in her Twitter posting. "I do LOOK pregnant in that photo!"
The "How I Met Your Mother" co-star is married to actor Alexis Denisof. They are the parents of a 2-year-old daughter named Satyana.
Hannigan and Denisof appeared together on the TV show "Buffy the Vampire Slayer."