Danes, 39, made the announcement on The Howard Stern Show last night, E! News reports.
"I'm pregnant," she said, adding. "I'm seriously preggo. I'm deep into my second trimester."
This will be Danes' second child with husband Hugh Dancy. They already have a five-year-old son, Cyrus.
Danes and Dancy have been married since 2009. Congratulations to the couple!