Travolta confirmed the couple's happy news last month (May10), just over a year after 16-year-old Jett died after suffering a seizure during a family holiday in the Bahamas last January (09).
The stars, who are also parents to 10-year-old daughter Ella, recently branded the pregnancy a "miracle" because they had been trying for "several years".
And Preston's mum, Linda Carlson, admits she "burst into tears" when she heard the happy news.
She tells UsMagazine.com, "I cried so hard I couldn't see straight, just bawled my eyes out. Because I've been so worried about her! When they're that unhappy... I could cry right now remembering..."
And Carlson reveals the pregnancy is helping the couple put their grief behind them: "I've been praying that this would happen. Especially since Jett's death. Anything to get them out of their sorrow. I know Kelly has been longing for another baby. She just loves babies... It's taken them out of such deep grief and made them able to focus on new life. It just couldn't come at a better time."
But the Travoltas are keen to keep Jett's memory alive and are planning to teach the new addition to their family all about his or her older brother.
Carlson adds, "He'll see the pictures and he'll be told stories. He or she, whatever. It couldn't come at a better time. Her aura is so filled with peace. It's changed their lives."