The winner of seven consecutive Tour de France races told People.com he and Hansen "are thrilled to confirm" the impending birth.
Adding they are "ecstatic and grateful," Armstrong's statement said: "We are very much looking forward to what 2009 brings on many fronts."
Armstrong, 37, has three children with his former wife -- a 9-year-old and 7-year-old twins -- all conceived with sperm frozen before he underwent chemotherapy for testicular cancer.
He announced earlier this year that he plans to return to bicycle racing as well as the Tour de France in 2009.
Notable deaths of 2014 [PHOTOS]