Coon, who also starred on "The Leftovers," first debuted her baby bump when she attended the Critics' Choice Awards on Jan. 11 alongside Letts, 52. But she waited until Tuesday to confirm her pregnancy.
The couple first met while working at the Steppenwolf Theatre Company in Chicago, where Letts debuted his 2008 Pulitzer Prize-winning play, "August: Osage County."
They tied the knot in 2013 after appearing together in the Broadway revival of "Who’s Afraid of Virginia Woolf?" Letts won a Tony award for his portrayal of George.
The couple is having a great year. In addition to starring together alongside Meryl Streep and Tom Hanks in "The Post," Letts earned a SAG nomination for outstanding performance by a cast in a motion picture nomination for his role in "Lady Bird."