Babies born just weeks before they are due may face an increased risk for cognitive and emotional problems, a new study suggests. 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.

"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times more likely at age 6 to have lower IQs, as well as higher levels of attention problems and symptoms of anxious, withdrawn behavior," said study researcher Nicole Talge, a postdoctoral researcher at Michigan State University, in a statement.

Late-preterm babies are those born between 34 and 36 weeks of pregnancy; full-term pregnancies last at least 37 weeks.

Talge 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 study falls in line with previous work, and adds to the evidence by looking at other potential causes of the problems encountered by these children, the researchers said.

"Previous studies reveal that babies born a little early are at-risk for short-term medical problems and possibly long-term behavioral and cognitive problems ," Talge said.

Though the increased risk for problems was seen at age 6, the researchers said, it is important to notice if these associations persist later in life, because children of that age are still developing.

The researchers collected data on babies born between 1983 and 1985 in urban and suburban areas of southeast Michigan. They compared late-preterm babies to full-term babies when the children reached age 6.

The results could be due to the effects of birth complications , neonatal complications, what the babies are fed or a combination of factors, Talge said.

It's important to untangle these factors because more children are being born late-preterm: The rate of such delivery has risen 20 percent since 1990. While there is debate as to why that is occurring, Talge said, the impact it is having needs to be addressed.

The study will be published in the December issue of the journal Pediatrics.