"(Early) Happy Birthday to me! And baby makes 3!" she said via Twitter Tuesday.
She and musician Mark Wilkerson married in 2003. They are already parents to sons Braydon Hart, 4, and Mason Walter, 6.
No word on the baby's gender or due date.
Hart is best known for her work on TV's "Melissa & Joey," "Clarissa Explains It All" and "Sabrina, the Teenage Witch."