How Much Higher Are My Preterm Baby's Risks of Health and Cognitive Problems?

My baby was born at 35 weeks, after I had premature rupture of the membrane (my water broke early) after an otherwise healthy pregnancy. I have read a lot of vague references to babies born at 35 weeks/late preterm being at a higher risk of learning disabilities/ADHD/autism etc. but am having trouble finding the specifics backing this up. How strong is the evidence of a connection here, and how much greater are the risks for a late preterm vs. full-term baby?
—Worried New Mom

This question is an active area of research, because the largest share of preterm babies are in this gestational age range. It’s well understood that very preterm infants are more likely to have learning differences, but this late preterm period is less clear.

There are a number of systemic reviews that evaluate this, more or less by looking at papers that compare cognitive outcomes for infants born in the window between 34 and 36 weeks (late preterm) relative to those born after that. As you might imagine, these analyses are somewhat challenging because there are some correlates of preterm birth that themselves may correlate with later cognitive performance.

But taking them at face value, the evidence argues for, at most, very small impacts. Some reviews (example) suggest there are no measurable IQ differences. Others (example) argue there may be small and possibly transient differences in certain measures of executive function or behavior.

Perhaps the most important takeaway from the literature is that even when it looks like there are differences, they are small and they are dominated by the vast range in the data. To give a sense of this, I like the graph below, drawn from this paper. The paper compares cognitive test scores at age 6 for late preterm versus term infants, and the graph shows the full range (this is called a “box and whisker plot”). On average, the IQ score in the late preterm group is about 1.3 points lower, but there is effectively full overlap in the distribution. The clear conclusion there is that your child’s IQ — and everything else — is determined much, much more by everything else than by their gestational week of birth.