Do Smarter Babies Walk Earlier?

Do you remember when your little baby took her first step? How about when she sat without support?  How about standing with assistance? Yes, for many of us these "milestones" are not exactly burned in our brain, but a new study from the journal Pediatrics suggests that some of these milestones may be really important – not just for baby journals, but for childhood development. For the video version of this post, click here.

Here’s the deal. We've known for a long time that kids with severe developmental disabilities in childhood seem to meet some gross motor milestones later than expected. But that's looking at an extreme case. The question these researchers had was whether delayed gross motor development would associate with later childhood development in kids without developmental delay.

To answer this question, they turned to the Upstate KIDS study, a prospective cohort study of over 6000 babies born in the New York area.  The study focuses only on 501 of the children though – a subset who agreed to a follow-up examination at 4 years.  So, if you’re keeping score, we’re already looking at a group that is not representative of the population at large.

Based on logs the mothers kept, the researchers looked at when the child achieved certain gross motor milestones like walking. They looked at 6 milestones in all, and compared them to the total developmental score at four years of age. The findings were… subtle.

After adjustment for factors like maternal age, prematurity, and others, there was a statistically significant association between one of the six milestones - later standing-with-assistance and total developmental score. That total score is driven by 5 subcomponents, and when those were analyzed individually, later standing with assistance was associated with worse adaptive and cognitive development.

Similar results were seen in the subset of kids with no developmental disability – the subset, which, speaking editorially here, really should have formed the primary analysis of this study.

So… ok… should we panic if our kids aren't standing and walking like a bunch of little Rory Calhoun's? I'm not ready for that yet. For one, the authors don't appear to have accounted for the multiple comparisons evaluated here – so the marginally statistically significant result has a pretty high risk of being a false-positive. Second, it's not immediately obvious what you would do with a kid who stands with assistance 2.1 months later than the average. Stand them up more? Send them to a neurologist?

In the end, we’d end up giving moms and dads just one more metric to worry about in a world obsessed with measuring kids' performance at every turn. Or every step.

Birth at 41 Weeks = Baby Genius?

A study appearing in JAMA Pediatrics suggests that children born late-term have better cognitive outcomes than children born full-term. As if pregnant women didn’t have enough to worry about. For the video version of this post, click here.

Let’s dig into the data a bit, but first some terms (sorry for the pun). “Early term” means birth at 37 or 38 weeks gestation, “full term” 39 or 40 weeks, and “late term” 41 weeks. In other words, this study is not looking at pre-term or post-term babies, all of the children here were born in a normal range.

Ok, here’s how the study was done.  Researchers used birth records from the state of Florida and linked them to standardized test performance in grades 3 through 10. Compared to children born at 39 or 40 weeks of gestation, those born at 41 weeks got test scores that were, on average, about 5% of a standard deviation higher. To get a sense of what the means, if these were IQ tests (they weren’t) that would translate to a little less than 1 IQ point difference. Not huge, but the sample size of over one million births makes it statistically significant.

10.3% of those born at 41 weeks were designated as “gifted” in school, compared to 10.0% of those born at full-term.

Before I look at what might go wrong in a study like this – is the effect plausible? To be honest, I sort of doubt it. One week extra development in utero certainly will lead to some differences at or near birth, but I find it hard to believe that any intelligence signal wouldn’t simply be washed away amid all the other factors that affect developing young minds prior to age 8.

Now, the authors did their best to adjust for some of these things – race, sex, socioeconomic status, birth order, but it seems likely that there are unmeasured factors here that might lead to longer gestation and better cognitive outcomes – maternal nutrition comes to mind, for example.

We also need to worry about systematic measurement error. These gestation times came from birth certificate data – in other words, many of these measurements may have been some doctors best guess. If the dates were determined by ultrasound, larger babies might be misclassified as later term.  Also, I suspect that if conception dates weren’t well known, a lot of doctors filling out the birth certificate may have just written “40 weeks” to put something in that box.

The authors attempted to look just at women where the likelihood of prenatal care was high, finding similar results, but again, with the tiny effect size, any small systematic measurement error could lead to results like this.

The authors state that this information is relevant to women who are considering a planned cesarean or induction of labor. Currently, the American College of Obstetrics and Gynecology recommends “targeting” labor to 39-40 weeks to avoid some physical complications of late-term birth. In my opinion, having this study change that recommendation at all would be premature.