Full-term babies born a bit on the early or late side are at higher risk of cerebral palsy, according to a new study in nearly 1.7 million Norwegian children.

It is important to emphasize that the absolute risk is still very low and the vast majority of children being born some weeks away from 40 weeks (full-term) will not develop cerebral palsy, Dr. Dag Moster of the University of Bergen in Norway, one of the study's authors, told Reuters Health.

Cerebral palsy is a collective term for several disorders that involve the brain and nervous system that first appear in early childhood. It is the most common reason for disability in childhood and is thought to occur because the brain has been damaged during fetal development or early infancy.

According to the March of Dimes, two to three out of every 1,000 children born have cerebral palsy; the non-profit group estimates that there are 800,000 children and adults with the condition in the United States.

Preterm birth is well known to increase cerebral palsy risk, but most children with the condition aren't born prematurely, Moster and his colleagues point out in the latest issue of the Journal of the American Medical Association.

To investigate whether being born later might influence risk as well, they looked at nearly 1.7 million children born in Norway at 37 to 44 weeks' gestation between 1967 and 2001. A total of 1,938 of these children were known to have cerebral palsy.

The lowest risk of cerebral palsy, the researchers say, was seen in children born at term (40 weeks), with about one in every 1,000 of these children having cerebral palsy.

The risk of having cerebral palsy was higher with earlier or later delivery. The risk for children born at 37 weeks was nearly 2 in 1,000; it was 1.25 in 1,000 for children born at 38 weeks; 1.36 in 1,000 for children born at 42 weeks; and 1.44 for children born after 44 weeks.

The reason for these increased risks at 37 or 38 weeks' gestation, or at 42 weeks or beyond, are not clear, Moster said.

One possibility is that a newborn's brain may be more vulnerable if he or she is born shortly before or after the normal 40-week mark. An alternative explanation may be that fetuses prone to develop cerebral palsy have a disturbance in timing of birth making them more prone to be delivered either early or late, Moster said.

Until the biological reason for the link between pregnancy duration and cerebral palsy risk becomes clear, the researchers say, it would be hasty to assume that interventions on gestational age at delivery could reduce the occurrence of cerebral palsy.

Women having a normal delivery outside 40 weeks, Moster said, still have a very small risk...that their child will develop cerebral palsy.

SOURCE: link.reuters.com/vah58n JAMA/Journal of the American Medical Association, September 1, 2010.