07/19/2011 at 08:45 PM EDT
Appearing on The Tonight Show
on Tuesday, Jenna Fischer was asked by host Jay Leno if her parents were thrilled over their first grandchild.
"This is the second grandchild," The Office
star corrected, "but first grandson."
When Leno jumped on the word grandson
, Fischer confirmed that she and husband Lee Kirk are expecting a boy.
"This is the exclusive, Jay," says Fischer, 37, who appears in the upcoming film A Little Help.
"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.”
star, who announced
her pregnancy in May, had another scoop: the baby is due in the fall.
"September we’re in what they call the home stretch," she says.