"We're as happy as we were surprised by this wonderful news," the couple told the site in a statement. "We haven't slept since 2013 and welcome advice from all parents who survived Two Under Two."
Cook, 35, and Gillies, 38, are already parents to 15-month-old Charlotte. They have been married since 2004.
Gillies currently stars on The Originals. Cook was most recently seen on TNT's Perception.
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