"They track its progress in mommy's belly," he said Saturday while attending the Ante Up for Africa charity poker event in Las Vegas.
His wife, Luciana, is set to deliver the couple's fourth child this fall. They currently have three daughters: Alexia, 11, who is Luciana's child from a previous marriage, Isabella, 4, and Gia, 22 months.
The kids "are very excited" about another addition to the family, Damon, 39, said.
But being a family man and major motion picture star isn't easy for the former Sexiest Man Alive. Damon credits "a great wife" for helping him juggle his responsibilities.
"She really keeps it together," he said at the poker event at the Rio All Suites Hotel, hosted by Oceans 11 pal Don Cheadle. "We just talk it out and work it all out. You say, this is the job and this is the time commitment and this is where it will be. I’m a lot easier to hire if you’re doing movies in New York."