Emily Oster

10 min Read Emily Oster

Emily Oster

More on the CDC Developmental Milestones

Your questions answered

Emily Oster

10 min Read

Several weeks ago, I wrote about the CDC revisions to its developmental milestones. You can read the original post here. I got a lot of responses. One piece of this was people wanting more on topics like crawling, or receptive language development. I’ve got those on the list for future posts. But a large share of the responses focused on the expressive-language milestones, and in particular the issue of “50 words at 30 months.”

Is this the right milestone, and what does it mean? I collected feedback from Michael Frank, who runs the Wordbank that I referenced in the post; from some of the speech-language pathologists behind The Informed SLP (JoAnne Berns, Marie Bloem, Katherine Sanchez, and Karen Evans); and from the CDC authors of the journal article that informed these guidelines. Today I’m going to relay and try to organize some of that feedback. I do not think we’ll get to an answer that satisfies anyone, but I’m hoping to capture the tenor of the debate.

I’m going to start by going through a few general points of criticism, then provide the CDC response, and try to summarize.

Issue 1: Counting words is hard

In the last post, I talked about a primary issue in evaluating language development, which is that it is difficult for parents to recall all the words their child has, especially as they increase. When it’s just mama, dada, kitty, and shoes, it’s no problem. But if your child says 50 or 100 or 300 words, you will not remember them all.

This is why, when researchers evaluate this, they use a long list of words and ask parents if their child knows them. Those types of assessments are where the Wordbank data comes from. Michael Frank very kindly provided me with the graph below, which shows the expected count of assessed words by age in the general U.S. population.

Based on this data, a child at 30 months who produced only 50 words would be well under the 10th percentile.

But! Michael pointed out that the milestone isn’t about how many words kids have based on this assessment. The milestone is based on what parents say when asked how many words their kids have. People generally underestimate (Michael told me that in the 1870s people estimated that peasants only said about 200 different words. Wrong.)

Assuming some underestimation, maybe this all lines up. If you want to identify kids at the 25th percentile at 30 months, and the assessment says 300 words, 50 words works if parents underestimate by a factor of 6. But there isn’t anything in the data that would point to this particular amount of underestimation.

Issue 2: The source of “50 words” is totally unclear

The conclusion of the above is that it is hard to measure parental-recall word production, which is really what the milestones are aiming to get at. This leads to the question of where, precisely, this number came from.

It is unclear.

The supportive publication references two actual data sources (and also a set of language scales). One of the sources is an aggregation of milestones but doesn’t have anything on 50 words. The second is a paper studying 40 children, looking at language development over the second year of life. This actually does look at parent-measured acquisition of 50 words. It finds an average of 18 months for this milestone, with a range of 13 to 21 months. There may be reasons, of course, why this doesn’t directly apply, but certainly this source wouldn’t justify the 50 words at 30 months.

I posed this question directly to the team at The Informed SLP, who said the following:

We have gone through virtually all the references to try to understand this and are coming up totally empty. We’re stumped. 

I am also stumped. More on the CDC response below, but I would say that the answer isn’t that we are missing something. There is no particular source that generates this number.

Issue 3: Single words is a poor milestone at this age

The most constructive criticism here is that it doesn’t make sense — perhaps for the recall reasons discussed above — to use word counts at this age. The Informed SLP team wrote me the following:

The separation of 50 words (at 30 months) from two-word phrases (at 24) is especially confusing. All of Zubler et al.’s sources that discuss both word combinations and a 50-word vocabulary have those two achievements at the same age (i.e. 24 months), which makes sense since you really don’t see kids building phrases before they have a 50-word productive vocabulary. A good reference for this (included in Zubler et al.’s reference list) is Fenson et al. (1994), which reports on the validation of the CDIs. We’ll include two very relevant passages from that publication here:

“If we accept a judgment of either ‘sometimes’ or ‘often’ as evidence for word combinations, then 20.9% of children with vocabularies under 50 words are already producing a few word combinations, and just under 50% of the sample are combining when vocabularies reach between 50 and 100 words. However, if we impose the more stringent criterion of combining ‘often,’ then only 1.3% of the children with productive vocabularies under 50 words and slightly more than half (54.1%) with vocabularies of 200-300 words meet the criterion.”

“The median number of words produced by children in the present study was 55 at 16 months, 225 at 23 months, and 569 at 30 months (note that this last figure is the most likely of the three to be an underestimate). Thus, the rate of vocabulary learning is 0.81 words per day between 16 and 23 months and (at least) 1.64 words per day between 23 and 30 months.”

To de-jargon a bit, the point here is that by this age, language learning is a lot about language construction and communication, and not about words per se. This would suggest that any expressive-language milestone at this age might be better based on some sense of communication ability rather than word counts. By the age of 3, the language milestone is along the lines of “can be understood most of the time.” It might be better to have a version of that at 30 months too.

Issue 4: There were no speech and language pathologists on the panel

This is a topic of some disagreement. The CDC says one of the paper authors was trained in both pediatrics and SLP. The Informed SLP team pointed out that this person isn’t working as an SLP and that it isn’t one of their main credentials.

As someone who is constantly being criticized for having the wrong credentials, I think this argument is basically unhelpful. The guidelines could be appropriate even if there were no SLPs, and they could be wrong even if there were. So I’m going to leave this one.

CDC response

I asked the CDC study authors about these issues. Because there has been so much criticism, I include my question and their verbatim response.

Is there a norming data sample that you can point me to that informed the 24-month and 30-month count of word guidelines? I see the citations in the paper, but they do not point to a particular data source. Is there something more specific that you can point me to? 

The CDC checklists are meant to be health communication tools for use at well child visits (health supervision visits). They are not to be used as developmental screening or evaluation tools. The revision process was undertaken because of feedback from parents, pediatricians, and early childhood professionals who felt the lists could be improved. Some areas that were identified were adding 15- and 30-month checklists, using clear language, not repeating milestones across checklists, and using milestones most children would be expected to achieve by that age so that developmental screening would more likely be done if a child is missing milestones or if parents or professionals had concerns (rather than waiting and seeing if the child develops the skill later on).

We used normative data that was published within a limited number of studies, and supportive evidence from common developmental screening tools and parent resources. We found normative data on individual milestones difficult to find. The group revising the milestones used that information along with clinical opinion to determine if the milestone should be included and at what age. Placing certain milestones on a specific age well visit checklist, when most children may achieve it between well visit ages, is something that can be reviewed as more evidence becomes available.

The group acknowledges (also in the paper) that there are limitations to the process and best practices for developmental monitoring/surveillance. They hope more research will be done to further identify best practices for developmental surveillance/monitoring.

I read this as acknowledging that the milestones are based as much on clinical judgment as on evidence.

The Informed SLP team told me:

When the authors say “evidence and judgment” led to their recommendations, but the sources they cite don’t check the “evidence” box, it leads us to believe that “judgment” was the deciding factor.

This seems consistent with the response.

Final thoughts

I will say that, academically, I find this debate interesting. It touches on why it’s hard to produce data. It also gets into how we translate evidence into practice. How do we take a word count from a developmental screening tool and translate that to “What do parents notice?” And even more, translate it into something pediatricians can implement at a 15-minute well-child visit, when they’re doing a million other things.

Looking at all of this together, though, it does feel to me like the 50-words milestone is a miss — or, maybe better said, a missed opportunity. It’s not that the count is right or wrong; it just seems like it isn’t really the right way to summarize the language development goals at this point. Evaluating language skills at this age is difficult, when a child is somewhere between “can put two words together” and “can talk.” Word counts don’t capture that, but it isn’t easy to see what specific guidance would capture it.

I asked the Informed SLP team what they’d want here, and they shied away from defining a milestone but said the following:

Not a word count, since it would be difficult for parents to estimate the number of words most children seem to have at this age. A more useful 30-month vocabulary milestone (if having one feels necessary) might be functional: something like “using words to communicate most all of the time,” “having a word for almost everything,” but there isn’t a ton of data to base this on. We don’t know of any other data sets looking at vocabulary size that could possibly have supported the recommendation. 

Perhaps a more important angle than vocabulary to consider at 30 months is emerging grammar and sentence length/complexity. SLPs evaluating children this age typically look at measures like the average length of the sentences children say, the emergence of grammar (e.g. tense and agreement), and the different types of word combinations children use. The work of Dr. Pam Hadley could be really informative here. 

My end guess is that the CDC will need to revisit this particular milestone. The authors’ response hints at a willingness to do so. There is an open question, though, of whether there are any simple guidelines for this age category that would be helpful.

Two women stand on a balcony chatting. One is pregnant.

Feb 27 2023

6 min read

Your Best Parenting Advice

ParentData is 3!

Emily Oster
A line graph with pink, yellow, and blue dots representing life's ups and downs.

Feb 21 2023

3 min read

Wins, Woes, and Autism

Your stories for the week

Emily Oster
A toddler sits on a couch poking at an iPad and smiling.

Feb 16 2023

4 min read

Infant Screen Time and Academic Success

Infant screen time and breakfast cereal terror

Emily Oster
A child sits on a couch playing with a fidget spinner. His mother, seated next to him, kisses him on the forehead.

Feb 09 2023

13 min read

ADHD Diagnoses in Children

Answering your questions with Erin O'Connor, EdD

Erin O’Connor

Instagram

left right
Do you brand things a certain way to get your kid to accept it? Like calling carrots “rabbit popsicles”? Or telling them to put on their “super speed socks” in the morning? Share your rebrands in the comments below! You never know who you might be helping out 👇

#emilyoster #funnytweets #relatabletweets #parentingjokes #kidssaythedarndestthings

Do you brand things a certain way to get your kid to accept it? Like calling carrots “rabbit popsicles”? Or telling them to put on their “super speed socks” in the morning? Share your rebrands in the comments below! You never know who you might be helping out 👇

#emilyoster #funnytweets #relatabletweets #parentingjokes #kidssaythedarndestthings
...

Have you ever panic-googled a parenting question when everyone else is asleep? If so, you’re not alone. 

Today is the first episode of a new biweekly series on my podcast: Late-Night Panic Google. On these mini-episodes, you’ll hear from some familiar names about the questions keeping them up at night, and how data can help. First up: @claireholt!

Listen and subscribe to ParentData with Emily Oster in your favorite podcast app 🎧

#parentdata #emilyoster #claireholt #parentingstruggles #parentingtips #latenightpanicgoogle

Have you ever panic-googled a parenting question when everyone else is asleep? If so, you’re not alone.

Today is the first episode of a new biweekly series on my podcast: Late-Night Panic Google. On these mini-episodes, you’ll hear from some familiar names about the questions keeping them up at night, and how data can help. First up: @claireholt!

Listen and subscribe to ParentData with Emily Oster in your favorite podcast app 🎧

#parentdata #emilyoster #claireholt #parentingstruggles #parentingtips #latenightpanicgoogle
...

Sun safety is a must for all ages, especially babies! Here are my tips for keeping your littlest ones protected in the sunshine:
☀️ Most importantly, limit their time out in hot weather. (They get hotter than you do!)
☀️ Keep them in the shade as much as possible when you’re out.
☀️ Long-sleeve but lightweight clothing is your friend, especially on the beach, where even in the shade you can get sunlight reflecting off different surfaces.
☀️ If you want to add a little sunscreen on their hands and feet? Go for it! But be mindful as baby skin tends to more prone to irritation.

Comment “Link” for a DM to an article on the data around sun and heat exposure for babies.

#sunsafety #babysunscreen #babyhealth #parentdata #emilyoster

Sun safety is a must for all ages, especially babies! Here are my tips for keeping your littlest ones protected in the sunshine:
☀️ Most importantly, limit their time out in hot weather. (They get hotter than you do!)
☀️ Keep them in the shade as much as possible when you’re out.
☀️ Long-sleeve but lightweight clothing is your friend, especially on the beach, where even in the shade you can get sunlight reflecting off different surfaces.
☀️ If you want to add a little sunscreen on their hands and feet? Go for it! But be mindful as baby skin tends to more prone to irritation.

Comment “Link” for a DM to an article on the data around sun and heat exposure for babies.

#sunsafety #babysunscreen #babyhealth #parentdata #emilyoster
...

I’m calling on you today to share your story. I know that many of you have experienced complications during pregnancy, birth, or postpartum. It’s not something we want to talk about, but it’s important that we do. Not just for awareness, but to help people going through it feel a little less alone.

That’s why I’m asking you to post a story, photo, or reel this week with #MyUnexpectedStory and tag me. I’ll re-share as many as I can to amplify. Let’s fill our feeds with these important stories and lift each other up. Our voices can create change. And your story matters. 💙

#theunexpected #emilyoster #pregnancycomplications #pregnancystory

I’m calling on you today to share your story. I know that many of you have experienced complications during pregnancy, birth, or postpartum. It’s not something we want to talk about, but it’s important that we do. Not just for awareness, but to help people going through it feel a little less alone.

That’s why I’m asking you to post a story, photo, or reel this week with #MyUnexpectedStory and tag me. I’ll re-share as many as I can to amplify. Let’s fill our feeds with these important stories and lift each other up. Our voices can create change. And your story matters. 💙

#theunexpected #emilyoster #pregnancycomplications #pregnancystory
...

OUT NOW: My new book “The Unexpected: Navigating Pregnancy During and After Complications” is available on April 30th. All of my other books came out of my own experiences. I wrote them to answer questions I had, as a pregnant woman and then as a new parent. “The Unexpected” is a book not to answer my own questions but to answer yours. Specifically, to answer the thousands of questions I’ve gotten over the past decade from people whose pregnancies were more complicated than they had expected. This is for you. 💛 Order now at my link in bio!

OUT NOW: My new book “The Unexpected: Navigating Pregnancy During and After Complications” is available on April 30th. All of my other books came out of my own experiences. I wrote them to answer questions I had, as a pregnant woman and then as a new parent. “The Unexpected” is a book not to answer my own questions but to answer yours. Specifically, to answer the thousands of questions I’ve gotten over the past decade from people whose pregnancies were more complicated than they had expected. This is for you. 💛 Order now at my link in bio! ...

OUT NOW: My new book “The Unexpected: Navigating Pregnancy During and After Complications” is available on April 30th. All of my other books came out of my own experiences. I wrote them to answer questions I had, as a pregnant woman and then as a new parent. “The Unexpected” is a book not to answer my own questions but to answer yours. Specifically, to answer the thousands of questions I’ve gotten over the past decade from people whose pregnancies were more complicated than they had expected. This is for you. 💛 Order now at my link in bio!

OUT NOW: My new book “The Unexpected: Navigating Pregnancy During and After Complications” is available on April 30th. All of my other books came out of my own experiences. I wrote them to answer questions I had, as a pregnant woman and then as a new parent. “The Unexpected” is a book not to answer my own questions but to answer yours. Specifically, to answer the thousands of questions I’ve gotten over the past decade from people whose pregnancies were more complicated than they had expected. This is for you. 💛 Order now at my link in bio! ...

OUT NOW: My new book “The Unexpected: Navigating Pregnancy During and After Complications” is available on April 30th. All of my other books came out of my own experiences. I wrote them to answer questions I had, as a pregnant woman and then as a new parent. “The Unexpected” is a book not to answer my own questions but to answer yours. Specifically, to answer the thousands of questions I’ve gotten over the past decade from people whose pregnancies were more complicated than they had expected. This is for you. 💛 Order now at my link in bio!

OUT NOW: My new book “The Unexpected: Navigating Pregnancy During and After Complications” is available on April 30th. All of my other books came out of my own experiences. I wrote them to answer questions I had, as a pregnant woman and then as a new parent. “The Unexpected” is a book not to answer my own questions but to answer yours. Specifically, to answer the thousands of questions I’ve gotten over the past decade from people whose pregnancies were more complicated than they had expected. This is for you. 💛 Order now at my link in bio! ...

Is side sleeping important during pregnancy? Comment “Link” for a DM to an article on whether sleep position affects pregnancy outcomes.

Being pregnant makes you tired, and as time goes by, it gets increasingly hard to get comfortable. You were probably instructed to sleep on your side and not your back, but it turns out that advice is not based on very good data.

We now have much better data on this, and the bulk of the evidence seems to reject the link between sleep position and stillbirth or other negative outcomes. So go ahead and get some sleep however you are most comfortable. 💤

Sources:
📖 #ExpectingBetter pp. 160-163
📈 Robert M. Silver et al., “Prospective Evaluation of Maternal Sleep Position Through 30 Weeks of Gestation and Adverse Pregnancy Outcomes,” Obstetrics and Gynecology 134, no. 4 (2019): 667–76. 

#emilyoster #pregnancy #pregnancytips #sleepingposition #pregnantlife

Is side sleeping important during pregnancy? Comment “Link” for a DM to an article on whether sleep position affects pregnancy outcomes.

Being pregnant makes you tired, and as time goes by, it gets increasingly hard to get comfortable. You were probably instructed to sleep on your side and not your back, but it turns out that advice is not based on very good data.

We now have much better data on this, and the bulk of the evidence seems to reject the link between sleep position and stillbirth or other negative outcomes. So go ahead and get some sleep however you are most comfortable. 💤

Sources:
📖 #ExpectingBetter pp. 160-163
📈 Robert M. Silver et al., “Prospective Evaluation of Maternal Sleep Position Through 30 Weeks of Gestation and Adverse Pregnancy Outcomes,” Obstetrics and Gynecology 134, no. 4 (2019): 667–76.

#emilyoster #pregnancy #pregnancytips #sleepingposition #pregnantlife
...

My new book, “The Unexpected: Navigating Pregnancy During and After Complications” is available for preorder at the link in my bio!

I co-wrote #TheUnexpected with my friend and maternal fetal medicine specialist, Dr. Nathan Fox. The unfortunate reality is that about half of pregnancies include complications such as preeclampsia, miscarriage, preterm birth, and postpartum depression. Because these are things not talked about enough, it can not only be an isolating experience, but it can also make treatment harder to access.

The book lays out the data on recurrence and delves into treatment options shown to lower risk for these conditions in subsequent pregnancies. It also guides you through how to have productive conversations and make shared decisions with your doctor. I hope none of you need this book, but if you do, it’ll be here for you 💛

#pregnancy #pregnancycomplications #pregnancyjourney #preeclampsiaawareness #postpartumjourney #emilyoster

My new book, “The Unexpected: Navigating Pregnancy During and After Complications” is available for preorder at the link in my bio!

I co-wrote #TheUnexpected with my friend and maternal fetal medicine specialist, Dr. Nathan Fox. The unfortunate reality is that about half of pregnancies include complications such as preeclampsia, miscarriage, preterm birth, and postpartum depression. Because these are things not talked about enough, it can not only be an isolating experience, but it can also make treatment harder to access.

The book lays out the data on recurrence and delves into treatment options shown to lower risk for these conditions in subsequent pregnancies. It also guides you through how to have productive conversations and make shared decisions with your doctor. I hope none of you need this book, but if you do, it’ll be here for you 💛

#pregnancy #pregnancycomplications #pregnancyjourney #preeclampsiaawareness #postpartumjourney #emilyoster
...

We are better writers than influencers, I promise. Thanks to our kids for filming our unboxing videos. People make this look way too easy. 

Only two weeks until our book “The Unexpected” is here! Preorder at the link in my bio. 💙

We are better writers than influencers, I promise. Thanks to our kids for filming our unboxing videos. People make this look way too easy.

Only two weeks until our book “The Unexpected” is here! Preorder at the link in my bio. 💙
...

Exciting news! We have new, high-quality data that says it’s safe to take Tylenol during pregnancy and there is no link between Tylenol exposure and neurodevelopmental issues in kids. Comment “Link” for a DM to an article exploring this groundbreaking study.

While doctors have long said Tylenol was safe, confusing studies, panic headlines, and even a lawsuit have continually stoked fears in parents. As a result, many pregnant women have chosen not to take it, even if it would help them.

This is why good data is so important! When we can trust the data, we can trust our choices. And this study shows there is no blame to be placed on pregnant women here. So if you have a migraine or fever, please take your Tylenol.

#tylenol #pregnancy #pregnancyhealth #pregnancytips #parentdata #emilyoster

Exciting news! We have new, high-quality data that says it’s safe to take Tylenol during pregnancy and there is no link between Tylenol exposure and neurodevelopmental issues in kids. Comment “Link” for a DM to an article exploring this groundbreaking study.

While doctors have long said Tylenol was safe, confusing studies, panic headlines, and even a lawsuit have continually stoked fears in parents. As a result, many pregnant women have chosen not to take it, even if it would help them.

This is why good data is so important! When we can trust the data, we can trust our choices. And this study shows there is no blame to be placed on pregnant women here. So if you have a migraine or fever, please take your Tylenol.

#tylenol #pregnancy #pregnancyhealth #pregnancytips #parentdata #emilyoster
...

How many words should kids say — and when? Comment “Link” for a DM to an article about language development!

For this graph, researchers used a standardized measure of vocabulary size. Parents were given a survey and checked off all the words and sentences they have heard their child say.

They found that the average child—the 50th percentile line—at 24 months has about 300 words. A child at the 10th percentile—near the bottom of the distribution—has only about 50 words. On the other end, a child at the 90th percentile has close to 600 words. One main takeaway from these graphs is the explosion of language after fourteen or sixteen months. 

What’s valuable about this data is it can give us something beyond a general guideline about when to consider early intervention, and also provide reassurance that there is a significant range in this distribution at all young ages. 

#cribsheet #emilyoster #parentdata #languagedevelopment #firstwords

How many words should kids say — and when? Comment “Link” for a DM to an article about language development!

For this graph, researchers used a standardized measure of vocabulary size. Parents were given a survey and checked off all the words and sentences they have heard their child say.

They found that the average child—the 50th percentile line—at 24 months has about 300 words. A child at the 10th percentile—near the bottom of the distribution—has only about 50 words. On the other end, a child at the 90th percentile has close to 600 words. One main takeaway from these graphs is the explosion of language after fourteen or sixteen months.

What’s valuable about this data is it can give us something beyond a general guideline about when to consider early intervention, and also provide reassurance that there is a significant range in this distribution at all young ages.

#cribsheet #emilyoster #parentdata #languagedevelopment #firstwords
...