This will be the second child for Piper, 29, and her husband, actor Laurence Fox. They also have a 3-year-old son, Winston. She is 15 weeks pregnant.
Is Matt Smith leaving Doctor Who?
The joyful news initially made Piper emotional because she thought she'd have to withdraw from starring in Neil LaBute's upcoming play Reasons to Be Pretty at the Almeida Theatre in London. Previews were scheduled for Nov. 10 with the first official night on Nov. 17.
Besides playing former Doctor Who companion Rose Tyler, Piper is also known for playing Hannah in Secret Diary of a Callgirl.