Researchers at the Harvard School of Public Health reported on Monday that they have identified a number of perinatal and neonatal conditions that may be linked to autism, although the evidence is still too scarce to point to specific culprits, they said.
Autism refers to a group of developmental brain disorders that obstruct a person’s ability to communicate and interact socially. The condition ranges from severe cases of “classic” autism to mild forms like Asperger’s syndrome.
The Harvard researchers reviewed the results of 40 previously published studies, and found that a variety of factors around the time of birth have been linked to the risk of autism later in life.
These include low birth weight, certain delivery complications such as umbilical cord problems, fetal distress during labor and indications of “poor condition” in the newborn — such as respiratory or heart rate problems.
However, the studies often arrived at conflicting conclusions as to whether any single one of those factors was related to autism. Additionally, birth and newborn complications typically do not occur in isolation, but in combination, the researchers noted.
Indeed, in a complex disorder like autism it would be highly unlikely that any single birth factor would be the sole culprit, said study leader Hannah Gardener, currently a researcher at the University of Miami School of Medicine but who was with the Harvard at the time of the study.
During an interview with Reuters, Gardener emphasized that parents of children affected by any single factor identified in the study should not be alarmed.
“There is no single strong cause of autism,” said Gardener.
“It’s important that parents not worry about any particular one of these risk factors,” she said, adding that autism is generally thought to involve a complex interaction between genetic and environmental factors.
The current study highlights the importance of continuing to examine which environmental factors before, during or after birth may act together with genetics to cause autism, Gardener said.
In the United States, an estimated one in every 110 children has an “autism spectrum disorder,” she said.
Experts have long believed that genes play a vital role in the risk for autism — based primarily on two studies that showed that when one identical twin develops autism, the other has a high likelihood of also being affected.
Most studies have found less similarity between fraternal twins, which share only about half of all their genes. Identical twins share all of their genes.
However, a twin-based study published last week found that genes played a much smaller portion of the risk than previous studies had suggested.
The researchers estimated that environmental factors common to twins accounted for about 55 percent of the autism risk. However, they could not identify which specific environmental factors were at play.
That study underscores the need for further research into the role of environment in autism, Gardener said.
One problem in studies that have centered on factors at the time of birth is that they have typically involved relatively small groups of children, Gardener explained.
In the current study, Gardener and her team combined the results of 40 previous studies in something known as a meta-analysis. They found that a number of perinatal ““ meaning around the time of birth — and newborn factors were linked to autism, and that infants affected by those factors were more likely to develop autism than unaffected infants.
In addition to low birth weight, umbilical cord-problems and fetal distress, other factors that may be linked to autism include multiple births, birth injuries to the baby, maternal hemorrhaging during childbirth and anemia or jaundice in the newborn, the study found.
Another was a low Apgar score — a measure of a newborn’s general health that includes heart rate, breathing and muscle tone.
However, there was “insufficient evidence to implicate any one perinatal or neonatal factor in autism etiology,” the researchers said.
There was, though, some evidence that exposure to a “broad class” of these factors may contribute to autism risk, they wrote.
Yet, even where these associations occur, it is not entirely clear why they do. For example, in the case of low birth weight, Gardener said it is unlikely is a risk factor for autism, but rather a “marker” of problems in fetal development that could indicate anything from poor nutrition to genetic influences.
It is not clear which, if any, of those factors might in turn trigger or contribute to autism development. A significant obstacle is that researchers still do not know what biological mechanisms ultimately lead to autism, Gardener said.
However, the current study was able identify a number of birth factors that showed no relationship to the development of autism, such as the use of anesthesia, forceps or vacuum during childbirth, high birth weight and newborn head circumference.
The study was published online July 11, 2011 in the journal Pediatrics.
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