I never was popular at school.
But I wasn’t really broadly disliked, either. I existed by staying out of people’s way, not being overly interested in making myself stand out for anything, and certainly never inviting controversy or confrontation.
That last one remains true.
And yet – here I am. With things to say that are likely to spark disagreement – my least favorite thing.
Despite this, my hope in sharing my research, what I’ve learned and what I’m wondering, is that we can open a conversation around how to move things forward in ways that truly benefit children, families, and our broader society. I also really hope you’ll join in.
Essay #1.
I’m writing to the very early childhood industry.
The service providers who work with families, the publishers and writers who add books to the ever-expanding parent advice genre, to the influencers online who make a living telling us all what we should be doing with babies and toddlers, the pediatricians and the WIC staff, the child psychologists and the toy entrepreneurs.
We don’t know what we think we know.
In 2022, the Centers of Disease Control changed their developmental milestone checklist. Some milestones were removed (famously, crawling). Others were moved to later ages.
Based on our collective reaction to these changes, you would have thought the CDC cared nothing about children, was full of pencil-pushing idiots without an iota of actual lived experience with children, and was going to create huge, long-lasting damage to all of America’s future generations.
I’m a nerd by nature. And so I wanted to understand the why. Surely the CDC didn’t just arbitrarily change the milestones without good reason.
I read the paper detailing the reasons behind the change.
The CDC said they changed their milestones to represent when 75% of children are expected to have reached the milestone. Before, their checklist listed milestones for when 50% of children were expected to have reached it. The theory goes, the 75% threshold creates a smaller – more accurate net – for further evaluation, and reduces a “wait and see” approach.
Of course, they pointed out, the new guidelines are not perfect for capturing every child who needs evaluation, because development is more nuanced than that. But it was better than what had existed before.
I could make sense of that, but I just kept having this nagging thought….why did the CDC change their milestones now? And why didn’t I ever realize the the existing milestones were at a 50% threshold? Something felt…unanswered. So I kept digging.
In my search, I found a 2019 article detailing an effort by a group of pediatric specialists who compared 4 different (widely used) developmental checklists. One of these checklists came directly from the CDC (healthychildren.org), obviously prior to the changes made. These specialists wanted to get an idea of the milestones that were being measured by different checklists and get some agreement on when these milestones should be appearing. In other words, when we say “your child can do xyz by THIS age”….we need to be saying the same thing to parents. Our checklists should agree.
That study found clearly – the milestones did not agree. Like at all.
In fact, what they found was a total of “728 discrete developmental observations”… and when they compared those observations across checklists, they found only 40 – or less than 6% – agreed across checklists.
Less than 6% agreed across all 4 – very widely used – checklists.
12% of them agreed across 3 out of the 4 checklists.
In other words – depending on which checklist you use, a child may be developing as expected or be delayed. And the only thing that changed was the checklist.
Even more weirdly – these checklists used many of the SAME SOURCES to inform their milestone descriptions and age. (I checked those original sources by the way. I was astonished at the absence of cited data in them. Caring for Your Baby and Young Child from the American Academy of Pediatrics is a very popular one. Unfortunately, the reference list for that publication – at least the versions used to inform those checklists in 2019- is non-existent).
So – the same sources were used to inform these 4 checklists…and still only 5.5% of the milestones agreed across all 4 checklists?
I felt like the floor had fallen out from under me.
I had always, always, always assumed that the child development I had learned was based on solid science, and was true of human children across history and geography. That infants learn to roll, and sit, and crawl, and pull up to stand, and cruise and eventually walk – at about the same ages – regardless.
And now we couldn’t even agree on what milestones are on our most popular checklists? Let alone which ages they should show up?
(By the way – of the milestones that did agree on 3 out of 4 of the checklists…the vast majority of them were motor. We have much less agreement on communication, cognitive and social emotional skills).
This discovery shook my professional confidence deeply, and resulted in me spending the next several years digging into the existing knowledge base to help me understand where we’ve come from, and where we stand today. I’ve moved far beyond our own “early childhood development” literature – and dug into history, anthropology, sociology, neuroscience, economics, and other fields to paint a more complete picture.
And I’ve discovered the foundation of knowledge from which we practice is a lot less supported by rigorous science than I was ever taught to believe.
The early childhood industry must wrestle with this. We are proclaiming ourselves to be expert, offering advice to parents and guidelines for children. We have really, really good intentions. But we are doing so with incomplete, often scientifically shaky, information.
What now?
I’ll be distilling what I’ve discovered through these essays. These are not my original ideas, but rather a synthesis of what scholars in a variety of fields have learned and written. I often do not have answers to the tensions I see – my aim is not to be another expert in a field already over-run with them. Instead, I want to shine a light on knowledge we take for granted but shouldn’t.
In each essay I write, I’ll link the relevant sources as well as cite them at the end. I hope you’ll join this conversation.
“We are not what we know but what we are willing to learn together.” — Mary Catherine Bateson
REFERENCES
American Academy of Pediatrics. (2014). Caring for your baby and young child: Birth to age 5 (6th ed.). Bantam. (*note: there’s been 2 more editions since this one, but this is the version that was used in the 2019 article cited).
IDOB Committee. (n.d.). IDOB: The Index of Developmental Observations. Retrieved September 11, 2025, from https://www.idob.info/. (*note: a detailed database of the 728 developmental observations found by the committee comparing checklists)
Wilkinson, C. L., Wilkinson, M. J., Lucarelli, J., Fogler, J. M., Becker, R. E., & Huntington, N. (2019). Quantitative evaluation of content and age concordance across developmental milestone checklists. Journal of Developmental & Behavioral Pediatrics, 40(7), 511–518. https://doi.org/10.1097/DBP.0000000000000695 (pmc.ncbi.nlm.nih.gov)
Zubler, J. M., Wiggins, L. D., Macias, M. M., Whitaker, T. M., Shaw, J. S., Squires, J. K., Pajek, J. A., Wolf, R. B., Slaughter, K. S., Broughton, A. S., Gerndt, K. L., Mlodoch, B. J., & Lipkin, P. H. (2022). Evidence-informed milestones for developmental surveillance tools. Pediatrics, 149(3), e2021052138. https://doi.org/10.1542/peds.2021-052138 (pubmed.ncbi.nlm.nih.gov)
Hi Katie, thank you for putting this information in an accessible format, and provocative at that. The 2022 CDC paper was at least important for its stimulation of conversation. I think an important component you haven’t yet mentioned is that the 2022 CDC milestones were designed with primary care developmental “surveillance” (a type of screening) in mind, so for this purpose the 75% threshold makes sense to me. But an OT colleague describes to me how this threshold has made actual developmental assessment (by Early Intervention or other OT/SLP/PT) and parent counseling more difficult because 75% can be falsely reassuring, in this OT’s view. Thanks!
Hi! Thank you so much for reading, Dr. Sanders! You’re right, I didn’t clearly mention that the changes were primarily made for physicians to use to help with surveillance at healthy baby checks. I agree, the 75% makes sense in that regard, in an effort to minimize both the “wait and see” approach and having too wide a net capturing kids for evaluation. I also think – and will write about later – this 75% threshold on the CDC milestones only makes sense for children growing up in our particular culture. These milestones end up being useless without cultural context.
I’ve been thinking more about the last part of your comment, in terms of the concerns raised by your OT colleague. I’d be really interested to hear more about her experience. We may be experiencing different scenarios with parents and evaluation. That’s not surprising, because I find there’s a lot of variability in how different states approach early intervention. But here are my initial thoughts:
-My experience has been that developmental assessments have not been affected by the CDC changes at all (at least the ones I’m familiar with) – they are still using normed scoring and standard scores. Perhaps her state is using the CDC milestones alone to determine eligibility – if that’s the case, that’s not how they were intended to be used, as you noted.
-But more likely, I’d bet she’s talking about justifying to parents why their child may benefit from therapy. I could easily imagine that a developmental assessment or evaluation could indicate the need for therapy, even if the child is still within that 75% window on the CDC milestones. For instance, a child who is 14 months old is not going to be caught by the CDC screener for not walking, but the PT sees atypical movement patterns that indicate an underlying problem. I agree, this is a tricky thing – especially if a parent is not concerned, but the therapist is.
But I haven’t had any situation when a parent has used the CDC milestones to push back against a recommendation for therapy. Rather, they simply aren’t ready or convinced their child needs additional support.
-I’m not sure if I touched on her particular concern or not! It’s why I want to start conversations with people – not because I think I’m an expert, but because I’m troubled by some of what I’ve learned and want to hash it out with others who get it.
Again, thank you so much for engaging! If I missed the mark with what she’s talking about, let me know – I’d love to keep thinking on it.
Time to write your own book, Katie.
I’m afraid I’d be an editor’s nightmare with my stream-of-consciousness!