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.

Inducing labor at 39 weeks - is picking your kid's birthday worth it?


Few aspects of modern medicine engender as much controversy as our labor and delivery practices. Rates of early induction of labor vary widely from country to country – even from hospital to hospital. And while some randomized trials have demonstrated that induction of labor prior to 40 weeks gestation might have favorable effects for infants with certain conditions like large-for-gestational age, we really don’t have much data on the effects of induction of labor during a normal pregnancy. But a study appearing in the New England Journal of Medicine attempts to shed light on that issue.

For the video version of this post, click here.

Run out of 39 hospitals in England, this study randomized 619 women, all age 35 or older on their first pregnancy, to labor induction at 39 weeks, or usual care.

Why do this? Well, for one thing, induction of labor prior to the official due date is pretty common. There is, perhaps, a quality-of-life argument to be made about having the ability to more or less choose when to deliver a baby. There is also some observational data that suggests that the sweet spot for delivery is around 38-39 weeks. Prior to that, complications associated with pre-term infants go up, and much beyond that and you start to see other birth complications.

Now, this study was clearly too small to detect differences in rare outcomes like neonatal or maternal mortality, but there has been some concern that induction of labor might increase the rate of c-section.

This study saw no such increase. The rate of c-section was 32% in the induction group and 33% in the usual care group – not statistically different. There were also no differences in rates of assisted vaginal delivery or NICU admissions, and every child in the study survived to hospital discharge. One fact caught my eye, though, and I think it gives us insight into the main limitation of this trial.

There was no significant difference in birth weight between the arms of the study. You’d think that the early induction arm would at least have slightly smaller babies. But in reality, the arms just weren’t that different in terms of any measured variables. Why? Well, there were women in the usual care arm who went into labor at 38 weeks. In fact, only 222 of the 305 women in the induction group got induction of labor prior to 40 weeks of gestation, as the protocol specified.

This bias, which the authors half-jokingly describe as “non-adherence”, is due to the fact that randomization into the study could occur at any time from 36-40 weeks. If you wanted to really answer the question that the authors pose, you’d randomize everyone at 39 weeks, and if they were put in the early induction arm, induce them at or near the time of randomization.

So we need to interpret this study not as saying that early induction is safe, but that a plan for early induction is safe. This is a subtle difference, for sure, but an important one if you are discussing inducing a woman who has already hit the 39 week mark. Still, in my book, a small victory for patient autonomy is a victory nonetheless.