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The ABCs of MLU

Updated: Jul 23, 2023

Hi, welcome to this blog post! If you're here and you're not an SLP, do you know what MLU stands for? Did you click on this title because you're curious? Should I draw this introduction out as long as possible to maximize your time on this webpage before I give you the answer?

Of course not. Let's get to why you're really here. What is MLU?

Could it stand for "Magical Lollipop Universe?" That sounds amazing, but no.

What about, "Mischievous Little Unicorn?" Maybe my next children's book, but not quite.

"Mega Llama Uprising?" If only.

Okay, okay, I've led you on long enough. The truth is that MLU stands for something much more practical, but much less thrilling than Marvelous Linguini Underpants. In fact, what it stands for is actually an excellent tool for tracking your toddler's language development:

Mean Length of Utterance.

Still with me? Let's learn more.

Going Beyond Words

When we think about developmental milestones, we tend to think about one thing:


How many words is my child using? How many words are they putting together? Is it one, two, three, or more? Are they making full sentences yet?

If you've read our other blog posts, you'll already know that there are a ton of prelinguistic milestones that are really important to think about before you even get to words (check out Tired of Waiting, Where's My Manual? and The Unspoken First). Now I'm here to tell you that once you do get to words, there are other subtle things we can look at to make sure your little one is on track. And although "Mean Length of Utterance" might sound like "How Many Words Can Your Child Combine," it's actually a little more complicated than that. It involves...


WAIT! Don't run! I promise this is won't be like your middle school English class where you had to learn about subjects and predicates and complex-compound sentences. This is way more simple. You got this.

The Nitty Gritty & The Mathy Bits

Are you ready to learn the nitty gritty? Do you like mathy bits?

Well, we calculate MLU by finding the average number of morphemes that a child uses across utterances. Both words and grammatical markers serve as morphemes. Time for some definitions if you don't already know these terms:

Grammatical Marker

A grammatical marker is added to a word in order to change its meaning. For example, you'd add to a verb if you wanted to change the tense. If I want to say I'll cook dinner tonight, I'll use the word "cook" with no grammatical markers. If I want to say I already cooked dinner last night so it's your turn, I'll use the word "cooked" with the grammatical marker for past tense, "-ed." If I want to say fine, I'm cooking now, I'll use the word "cooking" with the grammatical marker for present progressive tense, "-ing."



Okay, now you have the building blocks! Let's put it all together with an example.

Let's say I'm seeing a little girl, Molly, for speech-language therapy. She's almost three years old (33 months) and just went through a language explosion. We were working on combining words into two-word combinations, but she's blown past that -- now she's combining two words ("Up mommy"), three words ("Daddy blue truck"), even four words ("I no want bed"). Her parents are thrilled and are asking if it's time to discharge. But before we do that, I want to take a more sophisticated measure of Molly's language complexity -- because I know that there's more to language development than just words.

I take a language sample for Molly. Each row is a separate utterance, and I score each utterance with how many morphemes it contains:



# of Morphemes



Doggie's milk

Doggie + 's + milk




He baby doggie

He + baby + doggie


He drink baby milk

He + drink + baby + milk




Mama do

Mama + do


No mine

No + mine


My doggie

My + doggie


Two doggies

Two + doggie + s


Normally, I would take a language sample with at least 50 utterances, but for simplicity's sake we'll leave it at 10. Next, I do the mathy bit.

[Total # of Morphemes] ÷ [Total # of Utterances] = MLU

Which in this case is:

22 ÷ 10 = 2.2

Okay, we've got our Magical Lion Umbrella, our MLU. That's how we do it. Now...what does it mean?

What MLU Can Tell Us

MLU is an excellent tool for an SLP because it gives an authentic average.

Typically when children start using four-word sentences like "He drink baby milk," that's all you pay attention to. Of course we want to share the exciting, brilliant, lengthy thing our child just said the other day. And we should celebrate that, that's amazing and wonderful!

But MLU ensures that we are looking at all of the language a child produces, not just those impressive few utterances.

Molly's MLU is 2.2. This tells me that, although she is using some longer utterances, she still uses mostly 1 and 2 word combinations. Also, she's probably missing some grammatical markers that would make these shorter combinations more complex. On a closer analysis I find that:

  • She is using possessive 's ("Doggie's milk")

  • She is using plural -s ("Two doggies")

  • She is not using is or contractions like he's ("He baby doggy")

  • She is not using present progressive -ing or present -s to conjugate verbs ("He drink baby milk")

  • She is not using articles like the or a to connect words

Molly is starting to use some grammatical markers, but there are still many she hasn't developed yet. She also hasn't begun using grammatical words, such as copulas (like "is") or articles (like "the"). By Molly's 33 months we want to be seeing more grammatical markers paired with a greater frequency of 3+ word combinations. So it looks like we'll keep supporting Molly for a little longer yet.

Okay, but what do we do now? How do we use this information to help Molly progress?

Toddlers Learn Grammar Without Worksheets

It's true. Jealous? Me too. Toddlers learn everything the fun way: through observing, imitating, and, well, play! (What a simpler time.)

Let me talk you through my two favorite strategies for teaching children grammar.

One: Model

By modeling, I mean model language by using language.

But wait, it can't be that simple?

No, for real. It is. This is why we don't want to use simplified grammar when talking to children - they learn from all that grammatical information you're effortlessly modeling for them day in and day out.

For Molly, we'll choose a few grammatical targets to model with a little more intention. First, we'll focus on present progressive -ing. While we play with Molly, or as parents go about daily activities (bath, lunchtime, errands), we'll make sure to use lots of present progressive verbs. And we'll emphasize them for good measure!

"I'm washing the dishes. Wanna help? Oh good, you're helping me wash the dishes! Look at you helping. You're helping and I'm washing."

Bonus tip: Modeling within routines can work wonders. Like in the above example, let's say dad washes the dishes every morning right after breakfast (if only, right?). Whenever he washes, he uses the same language "I'm washing the dishes and you're helping." Now Molly knows to expect these words within this routine, and is more likely to start using them herself.

Two: Recast

This is a lot like modeling, except that you're building on what the child has already said.

When Molly says, "I help daddy," dad can reply, "That's right, you're helping daddy." This is a way of correcting Molly without explicitly correcting her. She gets to feel validated for communicating, share a meaningful exchange with dad -- and hear that more accurate version of what she was trying to say.

If you're interested in learning more Parent Strategies, check out our Instagram for tons of educational posts (tip: look for Strategy Spotlights in the Guides section!).

The Milestones

Okay, forget about Molly. She's cute and all, but what about your own kid? How do you know when to be concerned? Should you calculate and track MLU yourself?

No! If you're concerned, seek an evaluation with an SLP. There's a lot more to language than MLU, and you'll want to make sure the professional is doing the heavy lifting. Your job is to parent, and that's hard enough as it is.

But, for your ease of mind, we'll talk expectations.

One easy rule of thumb is that children should be using an average of 2 words per utterance by 2 years, 3 words per utterance by 3 years, and 4 years per utterance by 4 years (remember: it's an average across all utterances).

But we know that MLU goes beyond words, so what about the grammatical complexity?

Well -- another easy rule of thumb is that, typically, all grammatical morphemes are acquired by 4 years of age. Before that there is quite a large range for when children develop each individual grammatical morpheme, but here is the general progression (Brown, 1973):

27-30 months

  • Present progressive -ing (e.g., jumping, racing, looking)

  • Prepositions in and on (e.g., on top, in cup)

  • Plural -s (e.g., kittens, trucks, toys)

31-34 months

35-40 months


If you feel like your child isn't quite meeting these expectations, seek support. (Check out our blog series on how to choose an SLP, starting with Setting the Scene, for more help on this).


Mean Loquacious Urials can help keep track of a toddler's development once they start using word combinations. This helps ensure that we look at language complexity rather than just utterance length. These are important aspects of language to consider when reviewing a child's development and how best to support them -- and when deciding if support is needed.

Two simple but effective strategies for supporting a child's increased MLU are to model and recast grammatical targets. If you're concerned that your child isn't meeting developmental expectations for MLU and grammatical complexity, seek an evaluation from an SLP.

(P.S. Just to clarify, it's actually called Mean Length of Utterance.)


Bowen, C. (1998). Brown’s Stages of Syntactic and Morphological Development. Retrieved from on July 18th, 2023.

Brown, R. (1973). A first language: The early stages. Cambridge, MA: Harvard University Press.

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