Emission Impossible is episode 11 of family guy season 03. It was originally aired on November 8, 2001.
Peter and Lois are inspired by her pregnant sister Carol to have another baby. Stewie is appalled at the prospect of another sibling and vows to keep Lois and Peter from sleeping together. After various attempts to stop them, Stewie shrinks himself down to a microscopic size and enters Peter’s body in hopes of destroying all of his sperm.