Autism seems to have a powerful genetic component, but a family history of the disorder isn't the whole story. The circumstances of a baby's birth may also predispose a child to developing autism, says a new study. Babies who come into the world after a difficult delivery, who have to be coaxed or sometimes pulled out of the birth canal, who have gotten tangled in the umbilical cord or whose first days are characterized by feeding problems, anemia or jaundice, these children face higher odds of developing autism than those whose births were more uneventful, says the study, published Monday in Pediatrics.
But a big head, high birth weight and a mom who got anesthesia during childbirth don't appear to raise the risk that a child will develop autism, adds the study.
Epidemiologists from Harvard University and Brown University conducted the "meta-analysis," which sorted through and combined the results of 60 studies that explored autism prevalence as a function of many birth and neonatal factors. Such analyses help discern how strong the links are between autism and each of those factors and allows researchers to rank how powerfully different factors can influence the risk of developing a given disorder. The same group of researchers established in 2009 that advanced maternal and paternal age was a strong prenatal risk factor for autism, as was a mother's bleeding during pregnancy and gestational diabetes.
The researchers, led by Harvard School of Public Health's Hannah Gardener, underscored that having a difficult birth may amplify a child's established genetic risk or may confer risk of the neurodevelopmental disorder all by itself. But they made clear that the baby who suffers a combinations of birth difficulties will likely have higher risks than a baby with only one. Their study comes in the wake of new research that downplays the role of genetics in autism.
"The obstetrical risk factors that have emerged as significant risk factors for autism in the current meta-analysis suggest a possible role of fetal and neonatal hypoxia," the authors note . A disruption of oxygen to the growing fetus seems to prompt a burst of activity in the cells that rely on the neurotransmitter dopamine, the authors note. And there is evidence that those with autism have unusual activity in those same cells.
The factors that appeared to contribute most strongly to a baby's autism risk were birth injury or trauma maternal hemorrhage, being four or more weeks premature at birth, birth weight of 2,000 grams (about 4 pounds, 6 ounces) or less, being in breech or another abnormal birth position, having a congenital malformation and having a low Apgar score at five minutes.