Recent research has compared the risk of cerebral palsy based on birth time and has concluded that delivery at 40 weeks has a lower risk of the condition than birth at 37 or 38 weeks or at 42 weeks or later.
"One of the strongest predictors of CP is preterm birth, with the risk of CP increasing steadily with earlier delivery. Although risk is lower among term births, about three-fourths of all infants with CP are born after 36 weeks," the authors wrote.
"Within this range of term births, there are few data on the possible association of CP with gestational age."
They used the Medical Birth Registry of Norway, which identified 1,682,441 children born in the years 1967-2001 with a gestational age of 37 through 44 weeks and no congenital anomalies.
Of the group of term and postterm children, 1,938 were identified as having cerebral palsy.
The researchers found that infants born at 40 weeks had the lowest risk of CP, with a prevalence of 0.99/1,000 births.
Risk for CP was higher with earlier or later delivery, with prevalence at 37 weeks of 1.91/1,000 (90 percent increased risk), prevalence at 38 weeks of 1.25/1,000 (30 percent higher risk), prevalence at 42 weeks of 1.36/1,000 (40 percent increased risk), and prevalence after 42 weeks of 1.44/1,000 (40 percent higher risk).
The authors added that these associations were stronger in a subset with gestational age based on ultrasound measurements, with a gestational age of 37 weeks associated with a 3.7 times higher risk of CP; and 42 weeks, a 2.4 times higher risk.
"Clinicians typically regard term births (37-41 weeks) as low risk, with the possibility of increased risk with postterm delivery.
Weeks 37 and 38 seem more to resemble weeks 42 and 43, both in CP risk and in the general likelihood of delivery, leaving 39 to 41 weeks as the optimum time for delivery," researchers wrote.
The findings were published in the journal JAMA.