The 37-year-old actress and her husband, graphic artist Peter White, are expecting a sibling for their four-year-old daughter Willow this autumn, reported Us magazine.
Monaghan met White at a party in 2000. They were married in August 2005 and live in New York.
In October 2011, Monaghan revealed that she had skin cancer. Her husband spotted a new mole on the back of her calf which turned out to be cancerous when she had it examined. There have been no further accounts of skin cancer since removal of the mole.