Babies who are born past their due date are at higher risk of cerebral palsy, say experts.
While premature birth is linked to the condition, little is known about the effects of being born over 40 weeks, which is classed as a full-term pregnancy.
Experts led by a team at the University of Bergen in Norway examined data for almost 1.7 million children born between 1967 and 2001.
The babies were delivered at any point from 37 to 44 weeks. The study followed the health of the children until 2005.
Of all the babies born at term or afterwards, 1,938 were identified as having cerebral palsy.
Analysis showed that babies born at 40 weeks had the lowest chances of cerebral palsy, but the risk was higher if the baby came earlier or later.
At 37 weeks, the risk almost doubled, and was 30pc higher at 38 weeks. At 42 weeks' gestation, babies were 40pc more likely to have the illness than those born at term, and the risk was also 40pc higher for those born after this time.
Cerebral palsy affects movement, posture and co-ordination and may be obvious at birth or may not become apparent until early childhood.