Newborns who needed support breathing—either oxygen or chest pumping—had a higher risk of having a lower IQ by age eight, even if they showed no signs of brain disease or impairment, according to a new analysis published online today in The Lancet. Babies who don't start breathing right away or have a low heart rate usually undergo some sort of resuscitation to ensure survival and adequate oxygen flow to the brain to prevent permanent damage.

The new findings are based on a review of the medical records of more than 5,800 babies born in the U.K. between 1991 and 1992. The researchers, from the Neonatal Intensive Care Unit at Southmead Hospital in Bristol, found that infants who were resuscitated at birth but did not show any symptoms of neonatal cognitive impairment had an absolute risk of 9.8 percent of having a low (80 or below) IQ score, compared with a 6.5 percent risk of the baseline group that didn't need—or receive—resuscitation. Those infants who received resuscitation and were still diagnosed with encephalopathy (brain disease) had a 23 percent absolute risk of having a low IQ by eight years of age.

The authors believe that the signs of subtler brain damage at birth (indicated by the need for resuscitation or caused by extra oxygen) might not be evident until later in childhood. "Physiological compromise before or during birth could damage areas of the brain that are critical for specific cognitive functions," the researchers write. They cite other studies that have shown lower performance on verbal tests of children who had required resuscitation just after birth, an indication that the prefrontal cortex (the area of the brain that processes higher order and abstract thoughts) might have suffered damage from inadequate oxygen.

The scientists say that several other factors that they did not control for may be to blame for the lower IQs, including maternal behavior during pregnancy (i.e. smoking or drinking) or the use of 100 percent oxygen for resuscitation, a practice, which although standard at the time of the study, has been phased out. A 2004 Lancet study noted that regular air (which is about 20 percent oxygen) was better for infants and that pure oxygen should only be used as a back up because its high level of free radicals can damage cells. 

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