Conglomeration of Pregnancy Conditions Might Increase Risk of Autism
As genetics alone is unlikely to be the only cause in the rapid rise in autism spectrum disorders over the past several years, researchers are looking more closely at environmental factors that could help explain the etiology of autism. A new study of more than 60 potential culprits has revealed 16 perinatal and neonatal conditions that might explain the increase in risk for an ASD.
Autism Etiology is Most Likely Multi-Factorial, with Both Genetics and Environment Playing Role
Hannah Gardener ScD, who was with the Harvard School of Public Health at the time of the study, conducted a literature review and identified 40 studies published before April 2007 that were suitable for further review and meta-analysis for autism risk factors. For each exposure, the research team calculated a summary effect estimate for each of 60 potential pregnancy and labor complications and found that 16 had significant associations with autism. Those with the strongest links include:
• ABO or RH incompatibility - babies with blood types incompatible with the mother had nearly four times the risk of developing autism
• Birth injury or trauma – associated with five times increase in risk
• Low birth weight (less than 3.3 pounds at birth) – tripled the risk of autism.
• Maternal hemorrhage – doubled the risk of autism
• Meconium aspiration – associated with sevenfold increase in risk.
• Neonatal anemia – associated with nearly 8 times the risk of autism
• Summer birth – associated with a 14% increase in risk, though researchers could not explain why
The remaining factors were linked to an increased risk, but to a lesser degree than those above.
• Abnormal presentation (ie breech)
• Congenital malformation
• Feeding difficulties
• Fetal Distress
• Hyperbilirubinemia (high bilirubin or jaundice)
• Low 5-minute Apgar score
• Multiple birth
• Small for Gestational Age
• Umbilical-cord complications
However, many of these complications are so closely related that they limit the ability to determine if any are independently associated with an increased risk of autism. For example, cesarean delivery is more common in pregnancies with abnormal fetal presentation, fetal distress, and multiple birth and is associated with a 26% increased risk. Congenital malformations, low birth weight and low Apgar are also interrelated and suggestive of problems with fetal development.
"Although there is insufficient evidence to implicate any one perinatal or neonatal factor in autism etiology, the studies using optimality scales provide some evidence to suggest that exposure to multiple neonatal complications may increase autism risk," wrote Gardener, whose work was published in the journal Pediatrics.
The analysis did rule out several factors that were suggested to be possible culprits. These include anesthesia, assisted vaginal delivery, post-term birth, high birth weight, and head circumference.
Autism Spectrum Disorders, which are now estimated to affect one in 110 children in the United States, are most likely of multi-factorial etiology involving an interaction between environment and genetics.
"There is no single strong cause of autism,” Gardener concludes. “This study suggests that several perinatal and neonatal complications may be related to autism risk, either alone, in combination or perhaps only in those who are genetically vulnerable."
Gardener H et al "Perinatal and neonatal risk factors for autism: A comprehensive meta-analysis" Pediatrics 2011; DOI: 10.1542/peds.2010-1036.
For more recent studies on the risk factors for autism, please read:
Environmental Factors May Significantly Impact Autism Risk
SSRI Antidepressant Use During Pregnancy May Increase Autism Risk