Willis, 58 and Heming, 35, who got married in 2009, are already parents to a 20-month-old daughter Mabel, reported Us magazine.
Willis, Heming and their daughter most recently were seen visiting a local farmer's market in LA. The 35-year-old model showed what appeared to be growing baby bump during the outing.
In addition to Mabel, the "GI Joe: Retaliation" actor is a parent to 23-year-old Rumer, 20-year-old Scout and 18-year-old Tallulah with ex-wife Demi Moore. Willis and Moore separated in 2000.
Willis is currently filming action movie "The Prince" alongside John Cusack, Jason Patric and 50 Cent.