Spending extra week in womb can make kids smart but physically disabled

Babies born late were also 2.1% more likely to require special classroom accommodation

Spending a week longer in the womb may give babies a tiny leg up on mental ability but put them at slightly increased risk of having a physical disability, says a study.

Babies born at late term -- 41 weeks' gestation -- are slightly more likely to be classified as gifted and have higher standardised test scores than babies born at full term, or at 40 weeks' gestation, the findings showed.

However, babies born at 41 weeks also showed a slightly higher chance of having a physical disability than babies born at 40 weeks, the study said.

"What our findings suggest is that while 40 weeks remains the safest time for most babies to be delivered, in uncomplicated pregnancies, going another week seems to have beneficial effects on later performance in school," said Jeffrey Roth from the University of Florida College of Medicine in the US.

The findings were published online in the journal JAMA Pediatrics.

The researchers drew their results from 1.5 million Florida birth records between 1994 and 2002.

They linked birth certificate information to public school records from 1998 to 2013.

Children born at the 41st week were found to score somewhat better on tests given at third to eighth grade.

Late-term infants were 2.8% more likely to be classified as gifted and 3.1% less likely to have poor cognitive outcomes compared with full-term infants.

Babies born late were also 2.1% more likely to be classified as having a physical disability that requires special classroom accommodation.

These physical disabilities most commonly include speech pathologies as well as sensory disorders and orthopaedic conditions, and can include children being homebound or hospitalised.

"These results are modest, but still meaningful," lead author of the study David Figlio, director of the Northwestern University Institute for Policy Research in Illinois, said.

"While late-term gestation is associated with somewhat higher rates for physical problems, it's also associated with better cognitive outcomes," Figlio noted.