The Simpsons season 24 episode 3
Episode title: Adventures in Baby-Getting
When Marge's car falls into a sink hole, she buys the perfect replacement to fit her family of five. But she quickly grows to hate it because, as she tells a shocked Homer, the car would be too small if she wanted to have another baby. Meanwhile, Bart and his friends are determined to find out what secret Lisa is keeping when Bart finds her cryptic messages and follows her sneaking off downtown for covert meetings after school.