Can Louis and Peter actually understand what Stewie is saying? Family Guy.?

It depends on what the plot requires.

Sometimes they understand him and respond as though he was an adult, but more often than not, they hear him and understand, it just processes as though he's speaking baby talk.

But like I said, it depends on the plot of the episode at that given time.

Here's something from a site giving more detail about the situation with Stewie speaking and I'll post the link below. Hope this helps clear up some things!

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Communication with adults

Stewie's speech is not always understood or acknowledged by other characters. The show once references this in an example of metahumor at the start of the seventh season in Love Blactually. A discussion between Brian and Stewie reveals that characters either fully understand Stewie or only understand him to a degree that is related to the character's closeness to the 'main cast'. When considering talking to Loretta, Brian tells Stewie that she won't understand him due to her proximity to Cleveland, making her close enough to the main cast.

Brian Griffin always understands

Brian is, as of now, the only main character whose ability to understand Stewie is unambiguous; the two always converse normally. In "Running Mates", Brian briefly appears to hear Stewie's thoughts; this phenomenon has not been explored in subsequent episodes.[16]

In two episodes, another baby by the name of Olivia communicates similarly to Stewie, with the exception that she is taken seriously by adults. Also, Jillian (Brian's girlfriend of several episodes) appears to understand Stewie just as well.

Adults understand, but disregard Stewie

At the end of the episode "E. Peterbus Unum", the question of whether adults could understand Stewie is raised. In the DVD audio commentary,[citation needed] MacFarlane explains adults can understand him, but don't take him seriously, "sort of like... if a four-year-old who [could] talk told you to '**** off', you'd laugh, because it's cute." In this manner, characters acknowledge Stewie, but pay no mind to his often-insulting addresses, such as calling Peter "the fat man" and referring to his mother by her given name, Lois. His insults are occasionally heard, such as when Stewie tells Lois "Why don't you burn in hell?!" Lois replies, "Well, no dessert for you, young man." Stewie generally is insulting to both Meg and Chris, but does acknowledge them. He also is one of the few characters who calls Meg 'Megan'.[5]

In "Untitled Griffin Family History", it is clearly shown that the other characters can understand Stewie; when Peter states that he dislikes The Godfather to the point where he can never watch the ending, Stewie asks "Well, how do you know you don't like it if you don't give it a chance?", and Lois says, "I agree with Stewie, it doesn't seem fair."

In "Lois Kills Stewie", Peter is being held at gunpoint by Stewie as he shouts at him, after Peter did not comment on Stewie's macaroni picture of an owl. A short conversation is then heard where the two talk for a short time. This conversation is acknowledged by the rest of the family.

In Road to Germany, Quaohog pharmacist Mort Goldman learns of Stewie's intelligence and communicates with him as fluently as Brian throughout the entire episode. However, this is quickly negated at the end when Stewie throws the knowledgable Mort back into his time machine, destroys it, and accepts the "past" Mort as the one to continue the series with.

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I hope that helped clear up some things!!

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Stewie is only understood by Brian, the only other member of he cast that should not be able to talk. The creator Seth MacFarlane has said that the character of "Stewie is meant to represent the general helplessness of an infant through the eyes of an adult." The general idea is "what would a one year old baby say if he could talk". The diabolical nature and his early on plans for world domination are so far out of left field there is where the humor is. When he's ranting and telling people to go to hell the cast only hears a cranky crying baby. Most of the time the main cast only gets the "gist" of what Stewie is saying, as mentioned in season 7 episode 1 when he and Brian go to talk to Cleveland's ex-wife Loretta. Before ringing the doorbell they discuss whether or not she's going to be able to understand Stewie or would she just get the "gist of what he's saying. This is just one of the on going jokes in Family Guy. .

The only episode where they totally understand him is the 2 part episode where stewie kills lois and then ends up getting killed when he takes over the world. They do understand him, because we find out at the end that the whole thing was just a simulation. The only person who understands Stewie is Brian. Other times, the characters will seem to respond to Stewie, but its really just like anyone would respond to any baby when they are babbling.

Lois And Stewie

I don't know about Louis, but Peter,Lois, and the rest of the family can understand what Stewie is saying. Then again it is a cartoon.

Yeah, as other people said, I always wondered that.

My impression is, if he wants them to understand him he can communicate to them, but when he's saying things he doesn't want them to hear he says it in baby-talk, that unintelligible babble that most babies seem to talk in.

Also, it's Lois, not Louis, just pointing that out.

:3

Nope says it in an episode

Where they are meeting with the fat black girl to get back together with the black guy...

Think the episode is "LOve Blactually"

no only brian can understand it says it on the episode where brian loses his girlfriend to cleavland it came out in 2008

It's "Lois" and I've been wondering this myself. Sometimes it seems like they can and sometimes, it doesn't. I think they hear his snarky comments as baby gibberish.

I really don't know myself, It's never been really clear on whether they can understand him or not. It seems like sometimes they can and sometimes they can't.