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The only episode that comes close to matching that description is One Little Word but the Smiths don't "get a baby".Bullock leaves Avery Jr. with Francine so he can go out & meet women.
There isn't one.You're thinking of 'April in Quahog'. FOX released promo pics featuring The Smiths and The Griffins but the crossover turned out to be an April Fool's joke.Although Francine did recently appear in a mock American Dad intro w/ Joe Swanson in the place of Stan, in the episode 'Foreign Affairs'
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She doesn't take a bath, she gets in their hot tub, but anyway, it's Season Six|Episode Five White Rice
Genesis Invisible Touch
The Worst Stan
Bullocks to Stan
season 4 episode 64, where he dates his daughter.
Son of Stan | Season 6 | Episode 2
She Did. In the Episode where Stan Visits his Native American half-brother.
The episode is called Roger 'n' Me and is from the first season.
She voices a stripper in Stan Knows Best