The couple released a statement on their websites saying they "are expecting a new addition to our family."
Travolta's publicist, Paul Bloch, confirmed Wednesday that Preston is pregnant, The Associated Press reports.
Preston, 47, and Travolta, 56, have a daughter, Ella, who is 10.
The couple's son, Jett, died last year at age 16.
The family's two dogs were killed Thursday in an accident at Maine's Bangor International Airport."It's impossible to keep a secret . . . especially one as wonderful as this," the couple said in a statement. "We want to be the first to share this great news . . . Love, John, Kelly and Ella."