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.