We all know the physical and mental (really the same thing) problems that preemies are prone to. This study of 40,000 kids in Scotland found an increase in the number of kids identified as special needs with kids born even a week before the nominal normal of 40 weeks. For kids born between 37 and 39 weeks, the increase was 16%.
Still, the total of preemies and and just a little early come to about 9% of kids with special needs. So 91% of special needs kids are full-term, and you have to expect that some (maybe 91%?) of the other 9% would have had special needs if they had held out until 40 weeks, so I don't see anything here that would change our practices.
I wonder about kids born at 41 weeks. Or do they allow that now that we can induce pregnancy so easily.