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.

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.

Twin studies have shown that when one identical twin develops autism, the other has a high likelihood of being affected as well. Most studies have shown less similarity between fraternal twins.

But another twin-based study last week estimated that environmental factors common to twins account for about 55 percent of the risk -- but they could not weed out what those factors might be.

Gardener and her colleagues found that a number of factors were linked to autism, including multiple birth, birth injuries to the baby, problems with the umbilical cord, maternal hemorrhaging during childbirth, and anemia or jaundice.

While none of these alone could be linked to a greater autism risk, exposure to a number of those factors might have an impact — possibly due to general compromises to a newborn's health.
But even where associations exist, it's not clear why.

For example, Gardener said that while it's unlikely that low birth weight per se is a risk factor, it can be a "marker" of problems in fetal development — anything from genetic influences to dysfunction in the womb to poor nutrition.

In addition, researchers still do not know what biological mechanisms ultimately lead to autism.
That said, the current study did identify a number of birth factors with no relationship to autism, including the use of anesthesia, forceps or vacuum during childbirth, high birth weight and newborn head circumference.