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Jenna Fischer is having a boy

Jenna Fischer talks to Jay Leno on the "Tonight Show," July 19, 2011. NBC

(CBS) Jenna Fischer gave Jay Leno an "exclusive" when she stopped by his show on Tuesday.

"The Office" actress told the "Tonight Show" host that she and husband Lee Kirk are expecting a baby boy.

Pictures: Celebrity baby bumps
Pictures: Jenna Fischer

Fischer shared the news after Leno asked if her parents were thrilled over their first grandchild.

"This is the second grandchild," she corrected, "but first grandson."

"This is the exclusive, Jay," the 37-year-old said. "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 also told Leno that she's due in the fall.

"September we're in what they call the home stretch," she added.

Last week, the actress told New York magazine's Vulture blog that her pregnancy will be written into "The Office" - Pam and Jim will be expecting baby No. 2.

"Pam will be pregnant when the season starts," Fischer revealed. "Pam and Jim snuck away last season on Valentine's Day, and they had sex. The story is that they conceived this baby at that time and were keeping it a secret until we come back from the summer."

Fischer and Kirk married last July ("Survivor" host Jeff Probst officiated the ceremony) and announced in May that they were expecting their first child.