Children born at 41 weeks scored better overall than those born at 39 or 40 weeks.
Research shows that if an infant is born full term, they can expect to experience better health and cognitive functioning while growing up and as they become adults.
Children who are born late may have a higher risk of health complications around the time of birth, but there is little information about the long-term cognitive and physical outcomes of being born after full term.
David N. Figlio, Ph.D., from Northwestern University in Evanston, IL, and colleagues looked at the records for more than 1.4 million children in Florida.
All the children were single births, as opposed to twins, and 80 percent of them attended public schools. They were all born between 37 and 41 weeks of gestation. The year of birth ranged from 1994-2002.
Measuring cognitive and physical ability
The researchers looked at three school-based measures of cognitive levels and two measures of physical status. They carried out the data analysis from April 2013-January 2016.
The measures of cognitive outcome were:
- Scores from the average Florida Comprehensive Assessment Test in mathematics and reading at ages 8 through 15 years
- "Gifted" status, defined as having "superior intellectual development" and being "capable of high performance"
- Achievement of a poor cognitive outcome, defined as scoring in the fifth percentile of test takers, or else being exempted from the Florida Comprehensive Assessment Test due to disability.
The measures of physical outcome were:
- Abnormal newborn conditions
- Physical disabilities listed in school records
This included any Exceptional Student Education placement, for example, because of speech, orthopedic, or sensory impairment or conditions necessitating longs stays in the hospital or at home.
The team compared the outcomes for late-term children, born at 41 weeks, and those born full term, at 39 or 40 weeks.
Later birth indicates higher cognitive functioning
For all three cognitive measures, infants who were born late achieved higher scores than those born full term.
Results indicate that those born in week 41 had higher average test scores in elementary and middle school, a 2.8 percent greater chance of being classed as gifted, and a 3.1 percent lower probability of poor cognitive outcomes. However, they also had a 2.1 percent higher rate of physical disabilities at school age, and they were more likely to have had health problems at the time of birth.
The findings suggest that there may be a "tradeoff" between physical and cognitive outcomes in those who are born late, say the authors.
Those who are born late appear to have a greater risk of abnormal conditions at birth and physical challenges during childhood, but they also have an increased chance of higher cognitive functioning.
The authors note that the results could help parents and doctors when making decisions about inducing delivery.
"While this article does not constitute a course of action for clinicians, our findings provide useful long-term information to complement the extant short-term data for expectant parents and physicians who are considering whether to induce delivery at full term or wait another week until late term."
The study had a number of limitations.
These include the fact that there may be different ways of measuring term. A possible source of bias could be that the children all came from families where either English or Spanish was spoken at home. In all the pregnancies, antenatal care began in the first semester. Finally, some children - for example, cases where the mother was over 35 years at the time of birth - were not included.