The 37-year-old actress - who stars as Pam Beesly in the US adaptation of `The Office` - is expecting a baby with her screenwriter husband Lee Kirk in September and she let slip the sex of the couple`s tot during a recent appearance on `The Tonight Show` last night (19.07.11).
When asked by 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."
Jenna tied the knot with Lee on Independence Day last year (04.07.10) and her co-star Rainn Wilson - who plays Dwight Schrute in `The Office` - revealed it was the "most beautiful" wedding he had ever been to.
Writing on twitter at the time, he said: "Went to the most beautiful wedding I`ve ever experienced - Total Love, JennaFischer and Lee Kirk! (sic)"