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Tuesday, July 12, 2011

Another Study, Another Set of Uncertainties

Reuters reports:

Research has hinted that various factors around the time of birth may raise a child's risk of autism later in life, but there is still too little evidence to point to specific culprits, a U.S. study said.

Experts have long believed that genes play a key role in autism risk, but a U.S. study released last week found that genes appeared to explain a much smaller portion of the risk than previously suggested.

The latest study, a review published in Pediatrics of 40 previous studies, found that factors including low birth weight, fetal distress during labor and signs of "poor condition" in the newborn, such as problems with breathing or heart rate, have been linked to the risk of autism.

"There is insufficient evidence to implicate any one perinatal or neonatal factor in autism etiology, although there is some evidence to suggest that exposure to a broad class of conditions reflecting general compromises to perinatal and neonatal health may increase the risk," wrote Hannah Gardener, a researcher at the University of Miami who led the study when she was at the Harvard School of Public Health.

More detail from MedPage Today:

Collectively, the studies examined more than 60 perinatal and neonatal factors. Meta-analysis showed that 16 of the factors had significant associations with autism:

  • Abnormal presentation, umbilical-cord complications, fetal distress, birth injury or trauma
  • Multiple birth, maternal hemorrhage, summer birth, low birth weight
  • Small for gestational age, congenital malformation, low five-minute Apgar score, feeding difficulties
  • Meconium aspiration, neonatal anemia, ABO or Rh incompatibility, and hyperbilirubinemia

The magnitude of the risk conferred by any one factor ranged from a relative risk of 1.14 for summer birth (P=0.02) to 7.87 and 7.34 for neonatal anemia and meconium aspiration, respectively (P=0.02, P=0.001). Use of healthy versus abnormal control groups for comparison increased the effect size for all of the factors.

Additionally, the analysis ruled out several factors as possible etiologic culprits:

  • Anesthesia
  • Assisted vaginal delivery
  • Post-term birth
  • High birth weight
  • Head circumference

"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," the authors wrote in their discussion.

"However, the correlated occurrence of many of these complications limits the ability to determine which factors, if any, are independently associated with autism."

As an example of correlated occurrence, they noted that cesarean delivery is more common in pregnancies with abnormal fetal presentation, fetal distress, and multiple birth. Additionally, congenital malformations, low birth weight, abnormal presentation, and low Apgar are interrelated