The former Home and Away star, who already has 4-year-old daughter Mia and 17-month-old son Cruz with her tennis ace husband Lleyton, said that the unexpected news is "fantastic".
Hewit told The Daily Telegraph: "We were thinking we'd wait until Lleyton retires because travelling with two is plenty. It's a happy surprise though.
"It wasn't fully planned, but it's fantastic. We were always going to have a third at some stage so it was exciting news."