Lily Aldrin disappeared for a few episodes during How I Met Your Mother season 4, and the sitcom came up with a funny way to explain her absence.
The 20th episode of the season, “Mobius Designs,” was the beginning of Lily’s absence. She appeared in the cold open and then disappeared from the series until episode 23, “As Fast as She Can.” The reason for the absence was due to Hannigan’s maternity leave. The actress took time off from the series during the birth of her first daughter in 2009. She missed a few episodes before returning for the final two episodes of season 4. To further explain why Lily wasn’t around, the series adapted the cold open so that it expanded on a particular joke.
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At the start of “Mobius Designs,” Barney decided to tell Lily a dirty joke despite Ted and Marshall’s warning that the content was “guy funny.” Future Ted claimed that it was the dirtiest joke he ever heard and that was reiterated by the fact that it caused Lily to walk away and avoid the group for the next month. This, of course, was an added line to cleverly explain the character’s disappearance in the series. Before that, the sitcom came up with hilarious ways to cover up Hannigan’s pregnancy throughout season 4. In fact, co-star Cobie Smulders, who played Robin Scherbatsky, was also pregnant with her first child around the same time.
When Lily returned a few episodes later, she didn’t appear until the final sequence, also referred to as the tag scene. When she reunited with her friends, Lily claimed that she had time to think about the dirty joke and realized that it was actually funny. That was until Barney said something else offensive, forcing her to walk away again. This time, she didn’t disappear from How I Met Your Mother. The actress also didn’t take an extended leave when she was pregnant for the second time in 2012. Instead, her pregnancy was written into the show to pave the way for the birth of Marvin Eriksen, the first child of Lily and Marshall.