Developmental Milestones Aren't Deadlines

A closer look at what's missing from the tools we use to track early childhood growth

I’ve lost count of how many times I’ve referenced the CDC’s developmental milestone checklists in conversations with parents. For the most part, the checklists are helpful, and they do what they intend to do: they give parents and professionals a shared language for tracking early childhood growth, and they flag warning signs of developmental delays that may require further investigation or intervention.

Before I share the rest of my thoughts about these checklists, though, I want to be crystal clear: I’m not here to trash the CDC. They’ve been under a ton of unnecessary pressure these days, and they do important, thankless work. Their milestone checklists were created thoughtfully and deliberately in collaboration with the American Academy of Pediatrics, based on a thorough review of the literature on child development.

That said, I think we need to be careful about how we use these checklists, and we need to be honest about their limitations.

The first problem is that the milestones are presented without any indication of range or variability. There’s no measure of central tendency, no standard deviation, no percentile ranks. Each item is framed as a binary: “By 15 months, your child should…” But a milestone isn’t a deadline. And without more information, it’s hard to tell what’s truly concerning and what’s just slightly late.

Second, some of the research behind these checklists is old. Roughly a quarter of studies included in the lit review were published before 2000, which means that a good chunk of the data predates shifts in population norms. There’s something called the Flynn Effect, which basically means that average scores on cognitive tests rise over time. It’s not unreasonable to assume other developmental benchmarks shift over time, too. So it can be misleading to use data from the 1990s (or earlier!) to define what’s “typical” today.

Third, some of the studies included in the lit review weren’t conducted in the United States, which raises questions about cultural relevance. Development is context-specific. What’s encouraged and expected can vary a lot from place to place. A skill that’s common at age three in one country might show up closer to age five in another. So if we’re building tools to guide American caregivers, we should use American norms as reference points.

Finally, the guidelines stop at age five. I get that screening and intervention efforts are intense in early childhood, but some of the most uneven developmental shifts happen later: during school years, during adolescence, and even during adulthood. By stopping at age five, we miss a huge part of the picture.

So while I appreciate the important work coming out of the CDC, I discuss developmental milestones with context and caveats. Because development is complicated, and no checklist—no matter how well-sourced or well-meaning—can capture the full story.

***

<3