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Aug. 31, 2010 -- Babies delivered at 37 or 38 weeks -- or at 42 weeks or later -- are at increased risk for cerebral palsy compared to those born at 40 weeks, a study shows. Still, the absolute risk of developing cerebral palsy is considered extremely low.
The study is published in the Sept. 1 issue of The Journal of the American Medical Association.
The CDC estimates that one in 303 children have some type of cerebral palsy. Symptoms vary and may include movement problems, muscle stiffness, poor muscle tone, and spasticity. The symptoms are thought to result from injury to the brain as a fetus or early in infancy.
"The absolute risk is still?very low, and the vast majority of children being born some weeks away from 40 weeks will not develop cerebral palsy," says study researcher Dag Moster, MD, PhD, of the University of Bergen, Norway, in an email. "It would be hasty to recommend intervention on delivery time based on this study."
What's more, "women having a normal delivery outside 40 weeks still have a very small risk that their child will develop cerebral palsy," he says.
The researchers looked at how the timing of delivery affected cerebral palsy risk among 1,682,441 single births between gestational ages of 37 and 44 weeks with no birth defects in Norway from 1967 to 2001. Of these babies, 1,938 were diagnosed with cerebral palsy.
The risk was higher at 37 and 38 weeks and at 42 weeks or later, compared with full-term, 40-week delivery, the study showed. Cerebral palsy can't be diagnosed at birth, so researchers followed the babies through 2005 using various registries.
Specifically, babies born at 37 weeks had about a 90% increased risk for cerebral palsy, compared to babies born at term. Compared to babies born at 40 weeks, babies born at 38 weeks had a 30% increased risk for cerebral palsy and those born at 42 weeks had about a 36% increased risk for cerebral palsy.
This risk increased about 44% when babies were born after 42 weeks, the researchers report. These associations were stronger among babies whose gestational age was based on ultrasound measurements, which can be a more accurate way of dating a pregnancy.
Exactly what causes cerebral palsy is unknown, but risk is known to increase with complicated labor and delivery, including preterm delivery, which was reinforced in this study.
As to why post-term delivery may increase the risk of cerebral palsy, "one possible explanation may be that the neonatal brain is especially vulnerable the more the baby is born away from a gestational age of 40 weeks," speculates Moster. "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."