This occurs in season 6 of Family Guy.
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Season 2, Episode 19 - "The Story On Page One"
if you don't want spoilers don't scroll down. . . . . . . . . . . . . . . . . . . . . . . . first stewie kills Lois and then it doubles up with the 101th episode where Lois comes back from the dead and trys to kill stewie then stewie becomes the ruler of the world and then Lois goes all sexy commando and has a f*****g awsome fight with stewie ending up kicking the s**t out of him then it turns out it was all imagination and stewie says "if that's what happens if i were to kill Lois then i better not" and then credits roll
Season 8 | Episode 13: "Go Stewie Go"
Stewie Loves Lois was created on 2006-09-10.
Your question makes no sense with "go in Lois butt", as there is no such episode where 'Stewie goes into Lois' butt', however you may be thinking of Season Four Episode 26 Petergeistwhen the exit of the spirit world was Meg's rear end.