The couple married last October and are the parents of a 16-month-old daughter.
"I have been dreading this announcement," Lisa wrote on her blog, Raddest Mom.
"I needed some time before being able to say it. Jack and I lost our baby boy last week. Having a late-term miscarriage is by far the hardest thing either of us have ever had to go through," she wrote. "We appreciate all of your prayers and ask for privacy and respect during this time. It's important to stay hopeful and optimistic through life's toughest moments. We never know why things like this happen. All we can do is trust in God and know he has a plan for our lives."
Lisa, 26, later tweeted: "Cuddling my amazing, sweet hubby and shutting my eyes. Thank you all for the love and support. It means the world."
Jack, 27, was announced this week as a contestant for "Dancing with the Stars," which kicks off its new season Sept. 16.
Swim Week Miami Beach 2014 [PHOTOS]