Jason, 35, revealed their exciting news during an appearance on US chat show Chelsey Lately, adding that he'd "put a baby in [Jenny's] belly".
According to E!, Jason refers to his wife while chatting to host Chelsea Handler in tonight's episode, saying: "Her breasts are very big these days.
"Would you like to know why? Cause I put a baby in her belly!"
In true Jim Levenstein/ American Pie style, Jason then points to his crotch and declares: "It works!"
Meanwhile, 34-year-old actress Jenny has also shared a scan picture of their unborn child with fans on Twitter. Aw!
Jason and Jenny met on the set of rom-com My Best Friend's Girl in 2007 and tied the knot in April 2008.
See which other stars are expecting a baby in our gallery below!