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NEW YORK - Studies have hinted at various factors around the time of birth that may raise a child's risk of autism -- but there is still too little evidence to point to specific culprits, a new research review concludes.
Looking at 40 previous studies, researchers found that a range factors around the time of birth have been linked to the risk of autism later in life.
Those include low birth weight, certain delivery complications like problems with the umbilical cord, fetal distress during labour and signs of "poor condition" in the newborn -- such as problems with breathing or heart rate.
But the studies often came to conflicting conclusions as to whether any single one of those factors was related to autism. On top of that, the researchers say, birth and newborn complications generally do not occur in isolation, but in combination.
And in fact, in a complex disorder like autism, it would be very unlikely that a single birth factor would stand out as the key culprit, explained Hannah Gardener, a researcher at the University of Miami School of Medicine who led the study.
In an interview, she stressed that parents of children affected by any one factor identified in this study -- low birth weight or umbilical-cord problems, for example -- should not be alarmed.
"There is no single strong cause of autism," said Gardener, who was at the Harvard School of Public Health at the time of the study.
"It's important that parents not worry about any particular one of these risk factors."
Moreover, autism is generally believed to involve a complex interaction between genes and environmental factors.
The current findings, Gardener said, underscore the importance of continuing to study which environmental factors -- whether before, during or after birth -- may act in concert with genetics to cause autism.
She and her colleagues report their findings in the journal Pediatrics.
It's estimated that about one in every 110 US children has an "autism spectrum disorder."
The term refers to a group of developmental brain disorders that hinder a person's ability to communicate and interact socially -- ranging from the severe cases of "classic" autism to relatively mild forms like Asperger's syndrome.
Experts have long believed that genes play a key role in autism risk. That's based largely on twin studies showing that when one identical twin develops autism, the other has a high likelihood of being affected as well.
Most studies have found less similarity between fraternal twins. Unlike identical twins, who share all their genes, fraternal twins share only about half.
But a twin-based US study published last week found that genes appeared to explain a much smaller portion of the risk than previous studies have suggested.
The researchers estimated that environmental factors common to twins accounted for about 55 per cent of the risk.
But they could not weed out what those factors might be.
That study, Gardener said, highlights the need for further research into the role of environment in autism.
A problem in the studies that have focused on factors around the time of birth is that they have generally included relatively small groups of children, Gardener said. So for their study, her team combined the results of 40 studies -- in what's called a meta-analysis.
They found that a number of "perinatal" (around the time of birth) and newborn factors were linked to autism -- that is, infants affected by those factors were relatively more likely to go on to develop autism than unaffected infants.
Along with low birth weight, fetal distress, and umbilical cord-problems (the cord being wrapped around the baby's neck, for instance), other factors included multiple birth, birth injuries to the baby, maternal hemorrhaging during childbirth, anemia or jaundice in the newborn. Another was low Apgar score -- a measure of a newborn's general health that includes heart rate, breathing and muscle tone.
But, the researchers write, there was "insufficient evidence to implicate any one perinatal or neonatal factor in autism etiology."
There was, though, some evidence that exposure to a "broad class" of those factors may contribute to autism risk -- possibly reflecting "general compromises" to a newborn's health.
But even where associations exist, it's not clear why they do.
Take low birth weight as an example. Gardener said it is unlikely that low birth weight, per se, is a risk factor for autism. Instead, low birth weight is a "marker" of problems in fetal development -- it could indicate anything from genetic influences to dysfunction in the womb to poor nutrition.
It's not clear which, if any, of those things might in turn fuel autism development. A big obstacle, Gardener pointed out, is that researchers still do not know what biological mechanisms ultimately lead to autism.
That said, the current study did find that a number of birth factors showed no relationship to autism.
Those included use of anesthesia, forceps or vacuum during childbirth, high birth weight and newborn head circumference.