Fischer appeared on The Tonight Show on Tuesday and told host Jay Leno that her parents are preparing for the arrival of their "first grandson" in September.
The Office star went on to joke that she'd saved the announcement of the baby's sex specifically for her Tonight Show interview, teasing: "When I went to my movie premiere [for A Little Help] 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!"
Fischer and spouse Kirk announced the impending addition to their family in May, with the actress later claiming that she's suffered very few negative side-effects from her pregnancy.
The 37-year-old Fischer recently revealed that her Office character Pam will also be pregnant during the sitcom's ninth season.
The Office returns September 22 at 9pm on NBC.
Fischer's latest film A Little Help premieres in US cinemas on July 22.
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Watch a trailer for A Little Help below: