"We're so thrilled and happy to start a family," the couple said.
Hewitt, 34, began dating Hallisay 15 months ago after reconnecting on The Client List, on which he plays Hewitt's estranged husband. They previously worked together on the short-lived NBC series Love Bites. Prior to that, Hewitt dated her Ghost Whisperer co-star Jamie Kennedy after ending her engagement to actor Ross McCall.
Hewitt is reportedly three months along, according to Us Weekly which first reported the news.