The TV star is best known for her work on "Desperate Housewives," "Joey" and "The Sopranos."
She and musician Shooter Jennings have been a couple for nine years and announced their engagement in 2009. They are already the parents of a 3-year-old daughter.
A source told People.com de Matteo is due to give birth to their second child in the spring.
The baby's gender was not reported.