She said: "In the beginning, it was a lot of macaroni and cheese and Lucky Charms. Now it's a lot of strawberries. I've been really lucky. I've had a really easy pregnancy."
The 37-year-old star revealed she was expecting a boy in July.
When asked by talk show host Jay Leno whether her parents were pleased with the pregnancy, Jenna said: "This is their second grandchild, but first grandson.
"This is the exclusive, Jay. When I went to my movie premiere the other day a reporter asked me if I knew what I was having, and I said I do, but I'm not sharing that yet because I'm [going to] have Jay be the first to know."