Baby Not On Board is episode 04 of family guy season 07. It was originally aired on November 2, 2008.
After Peter gets sexually harassed by Chris, Peter wins a year of unlimited gas, and Brian suggests that the family use the funds to take a road trip to the Grand Canyon. So the Griffins pack up the car and hit the road, but leave some very precious cargo at home. When the family finally realizes that they have left Stewie home alone, it’s a mad, explosive, hilarious dash back to Quahog.