Preston, 47, is pregnant, the family tells PEOPLE.
"It’s impossible to keep a secret ... especially one as wonderful as this," they say in a statement. "We want to be the first to share this great news with everyone that we are expecting a new addition to our family.
Love, John, Kelly and Ella"
Travolta, 56, and Preston are also parents to Ella Bleu, 10. Their 16-year-old son Jett, died of a seizure in January 2009.
The family also shared the news Tuesday on Travolta’s Web site and Preston's site.