New research suggests that it's not just premature babies who are at risk of cerebral palsy; so are babies born past their due date.

A new study published in the Journal of the American Medical Association, finds that compared with healthy babies delivered at 40 weeks gestation, babies born as early at 37 weeks or as late as 42 weeks or later appear to have an increased risk of cerebral palsy.

Cerebral palsy is the most common cause of physical disability in childhood. The causes of CP remain largely unknown. While it's associated with complicated deliveries, most cases have little do with the actual birth.

One of the strongest predictors of CP is preterm birth, with the risk increasing steadily the earlier the delivery. And yet, about three-fourths of all babies diagnosed with CP are born after 36 weeks.

So, to better understand the risk, Dr. Dag Moster, of the University of Bergen, Norway, and colleagues examined the relation of CP risk with gestational age among term and post-term births using the Medical Birth Registry of Norway.

They looked at more than 1.6 million births between 1967 and 2001 with a gestational age of 37 through 44 weeks. Of the term and post-term children, 1,938 were identified as having cerebral palsy.

The researchers found that infants born at 40 weeks had the lowest risk of CP. They also found:

• a 90 per cent increased risk of CP at 37 weeks
• a 30 per cent higher risk at 38 weeks
• a 40 per cent increased risk at 42 weeks
• and a 40 percent higher risk after 42 weeks

The researchers said their found the age of the mother didn't matter, nor did the baby's gender, or various socioeconomic measures.

They also noted that while most doctors consider births between weeks 37 and 41 as low risk, in this study, they found that, in terms of CP risk, weeks 37 and 38 resembled weeks 42 and 43. That leaves 39 to 41 weeks as the optimum time for delivery.

The researchers say doctors need to better understand the "biological mechanisms" for the birth-CP risk patterns they saw.

They also note that it's possible that fetuses predisposed to CP have a disturbance in the timing of their delivery. If that's true, trying to regulate the timing of their birth might not reduce the occurrence of CP, they note.

"If infants prone to CP are disrupted in their delivery times, the prevalence of CP would be unchanged regardless of time of delivery," they write.