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.
Cerebral Palsy Risk
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."
Amos Grunebaum, MD, director of clinical maternal-fetal medicine at the New York Hospital-Cornell Weill Medical College in New York City, stresses that the risk of cerebral palsy is very low to begin with. "There is only a small risk of one in 1,000 births to begin with, and there are many different possible causes of cerebral palsy," he says.
"More often than not, cerebral palsy diagnosis is due to events preceding labor and delivery," he says. "There are certain conditions where the fetus won't go into labor naturally, and the women in the study who delivered after 42 weeks may have already had a baby with CP."
"Delivering a baby earlier doesn't prevent it, but certainly you don't want to deliver a baby after 42 weeks and not before 39 unless there are medical reasons for doing so," he says. "Delivering more than two weeks after your due date does increase the risk for complications in general."
"It's hard to know whether these relationships are causal or just reflect a biology in motion," says Dwight Rouse, MD, professor of obstetrics and gynecology at the Alpert School of Medicine at Brown University in Providence, R.I., and an attending physician in the department of maternal and fetal medicine at Women and Infant's Hospital of Rhode Island.
"Babies destined to be abnormal often don't deliver when they are supposed to," he tells WebMD. "The relatively higher risk at 37 and 38 weeks as opposed to 39 or 40 weeks may be one reason to avoid elective early delivery, [but] if there is a good reason to deliver at 37 weeks, this study should not alter that," he says.
"United Cerebral Palsy is looking forward to additional study about increased incidents of cerebral palsy in at term and post term births," says Stephen Bennett, president and CEO of United Cerebral Palsy in Washington, D.C., in an email. "Ongoing research about cerebral palsy and developmental disabilities is vital to helping to create a life without limits for people with disabilities. We applaud this ongoing research and hope that this study and others like it will continue to improve the lives of those with disabilities."
United Cerebral Palsy is a nonprofit group that advocates for people with cerebral palsy and other disabilities.