An increased risk of cognitive and emotional problems threaten late-preterm babies born between 34 and 36 weeks.
The findings held up even when the researchers accounted for the mother's IQ and other demographic measures known to affect the risk of these problems.
While late-preterm births (full-term pregnancies last at least 37 weeks) have been associated with such problems before, the study by researchers at the Michigan State University represented one of the most rigorous looks at the issue by accounting for other potential causes.
"Previous studies reveal that babies born a little early are at-risk for short-term medical problems and possibly long-term behavioural and cognitive problems," said study researcher Nicole Talge.
"We wanted to look at a diverse population of children and take into account important factors such as maternal IQ and birth weight for gestational age; do the previously reported associations still hold up?" he added.
"We found late-preterm babies are between two and three times more likely at age 6 to have lower IQs, as well as higher levels of attention problems and symptoms of anxious, withdrawn behaviour," said Talge.
The researchers said that not all babies born at the late-preterm stage have problems, and further research is needed into the factors contributing to the increase in problems.
The researchers analyzed data on babies born between 1983 and 1985 in urban and suburban areas of southeast Michigan and compared it with data collected when the children reached age 6.
For each late-preterm baby analyzed, a full-term counterpart was randomly identified as part of a control group, taking into account birth weight for gestational age.
"Children born late-preterm exhibited lower levels of cognitive performance and higher levels of behavioural problems at the age of 6 even after accounting for socio-economic factors and maternal IQs," said Talge.
The researchers said that the increased risk was found at age 6 and since children were still developing at that age, it was important to see if and how these associations persisted later in life.
The research is published in the current edition of the journal Pediatrics.