Back in September, the How I Met Your Mother actress denied she was pregnant again. However, her rep recently confirmed the news to Us Weekly.
"Alyson is thrilled to be pregnant!" a source claimed. "She's been wearing baggy clothes to cover the baby bump."
Hannigan is married to actor Alexis Denisof. The couple also have a 2-year-old daughter named Satyana.
She reprises her role in American Reunion opposite co-stars Jason Bigg, Tara Reid, Seann William Scott and John Cho, which is the latest instalment to the American Pie movie series.
Hannigan previously said: "It's been hilarious. It really did feel like a reunion. The great thing about that cast is we all had such great chemistry."
Watch the teaser trailer of American Reunion below: