On Thursday, the actress, 37, was photographed chatting on her cellphone in Santa Monica, Calif., wearing jeans, a vest and a thin red crocheted sweater -- snug enough to reveal an undeniable baby bump.
"Alyson is thrilled to be pregnant!" a source close to the star tells Us Weekly. "She's been wearing baggy clothes to cover the baby bump."
The star and her husband, Alexis Denisof, are already parents to daughter Satyana, 2.
Married since 2003, Hannigan and British actor Denisof, 45, met on "Buffy the Vampire Slayer" and its spin-off, "Angel."
"[Being a mother] has changed me in every way because my life is so much greater," Hannigan told Us Weekly in May 2010. "It filled a void I never knew was there -- it's the most amazing experience of my life. I just feel so blessed to get to be on this journey, to be a mommy. [Satyana] is just the greatest baby.
MORE FROM WONDERWALL: