There is the question of what it means to be early, on time and late. Most research into due dates shows that are usually very off and that the 40 week timeline can be very innaccurate. NICU rates have been rising due to doctors inducing or planning c-sections at 39 weeks because they think all babies are ready by then. This happened to my nephew. Then you hear of babies born much earlier than that (35 or 36 weeks) and are just fine.
I think early, on time, or late depends on the baby. If the baby is born when they're ready to be born and suffer no problems from that birth, then I would say they were on time.
Sorry -- I'll get off my soapbox now. Did waaaayyy too much research before my son was born.
For purposes of counting - mine were "on time." My son was technically 8 days after is his due date and my daughter was one day after. Not sure where they are on the IQ scale, but they're definitely giving me a run for my money.
I would also hazard to guess that breastfeeding, or at least a nurturing environment, and proper prenatal care have more of an impact on IQ (statistically over a large population) than when the child was born for anything besides severely premature.
I only breastfed my daughter for 8 weeks, but am practicing extended breastfeeding for my son. Prenatal nutrition was about the same for each.
