Duff, 23, married Ottawa Senators hockey player Mike Comrie, 30, Aug. 14, 2010.
"We also want to share the exciting news that… BABY MAKES THREE," People.com said she wrote in a weekend message about celebrating her first wedding anniversary. "We are extremely happy and ready to start this new chapter of our lives."
Duff did not say when the baby is due or what its gender is.
The actress is known for her television series "Lizzie McGuire," as well as the "The Lizzie McGuire Movie," "Cheaper by the Dozen," "A Cinderella Story," "Cheaper by the Dozen 2" and "Material Girls."