Cerebral palsy is the most common cause of physical disability in childhood and affects approximately 2 out of 1000 children. Varying in severity, cerebral palsy affects movement and posture and is caused by disturbances in brain development during pregnancy, childbirth or the first few years of life. It is well-established that pre-term delivery increases the risk of cerebral palsy, but the majority of children with cerebral palsy are not born prematurely. Information on the association between gestational age and cerebral palsy after the pre-term period is limited.
Dag Moster of the University of Bergen and colleagues studied the relation between gestational age and cerebral palsy risk using data from the Medical Birth Registry of Norway. They examined data for 1.68 million singleton births without congenital abnormalities with a gestational age of 37-44 weeks in Norway from 1967-2001 and then linked these to the National Insurance Scheme to identify subsequent cerebral palsy diagnoses.
The researchers noted that children born at 40 weeks had the lowest risk of cerebral palsy and those born pre-and post-term had a higher relative risk. At 38 weeks the risk of cerebral palsy was increased by 30% and at 37 weeks by 90%. The risk also increased with increasing gestational age from 40 weeks and was 40% increased after passing 41 weeks. These differences were even more pronounced in a subset where gestational age was based on ultrasound measurements.
Influences birth timing?
The biological mechanism behind this increased risk is unknown. One possible explanation may be that the neonatal brain is especially vulnerable the further away the baby is born from a gestational age of 40 weeks. An alternative explanation is that foetuses with a pre-disposition to developing cerebral palsy have a disturbance in timing of birth that makes them more prone to early or late delivery. The researchers demonstrated a similar association between gestational age and congenital anomalies which supports the latter explanation.
“Until the biological mechanisms are better understood, it would be hasty to recommend intervention on delivery time based on this study,” said the researchers.
Moster, D et al. Cerebral Palsy Among Term and Postterm Births. JAMA September 1, 2010, Vol 304, No. 9