Study: Factors Related to Oxygen Deprivation, Fetal Growth May Be Associated With Autism
By Brenda Goodman
WebMD Health News
Reviewed By Louise Chang, MD
July 11, 2011 -- Several problems that crop up during labor and shortly after birth appear to increase a child's risk for developing autism, a new study shows.
The study comes on the heels of new evidence suggesting that environmental and developmental exposures may play a greater role in the development of autism than previously believed. For decades, doctors thought genes accounted for as much as 90% of all autism risk.
The study, a review of 40 studies published before April 2007, looked at a host of circumstances that may affect babies during labor and delivery. It found 16 circumstances that appear to be tied to a significantly increased risk that a child would develop autism later in life.
Researchers note that many of these complications tend to occur together in difficult or high-risk deliveries, making it difficult to finger a single suspect.
But broadly, researchers note, they seem to be related to oxygen deprivation and growth retardation.
“Reduced oxygen supply, during labor, during delivery, during the prenatal period, during early infancy, could influence autism risk,” says study researcher Hannah Gardener, ScD, an epidemiologist at the University of Miami's Miller School of Medicine. “We can't say that definitely from our study, but that certainly is one possibility.”
Complications Related to Autism
Neonatal anemia, a condition in which the body doesn't have enough oxygen-carrying red blood cells, for example, was associated with nearly eight times the risk a child would develop autism later in life.
Meconium aspiration, which can occur when a fetus that's under stress and not getting enough oxygen inhales waste products inside the womb, was linked to a sevenfold increase in the likelihood that a child would later develop autism. Birth injury or trauma increased autism risk fivefold.
Babies with blood types incompatible with their mother's had nearly four times the risk.
Very low birth weight infants, or infants weighing less than 3.3 pounds at birth, faced triple the risk.
Maternal hemorrhage more than doubled the odds.
Other factors tied to increased autism risk, though to a lesser degree, included congenital malformations, breech and other kinds of abnormal birth positions, multiple birth, a low 5-minute Apgar score, weighing less than 5.5 pounds at birth, umbilical cord complications, fetal distress, being small for gestational age.
Being born during the summer months was associated with a very slight, 14%, increase in the risk of developing autism, and researchers say they can't explain why birth season might play a role.
Cesarean delivery was associated with a 26% increased risk. But that risk was weaker, statistically, than the other exposures identified by researchers, so they aren't sure it's a true association. It is likely, they say, that babies who are having trouble and under stress are simply more likely to be delivered by C-section, which could explain the trend.
Factors not associated with autism risk included the use of anesthesia during delivery, assisted vaginal delivery, being born late, high birth weights, or the size of a baby's head.
Study May Guide Future Research
“I think that we are just beginning to appreciate the potential role of events in-utero on autism risk,” says Linda Dodds, PhD, professor and director of research in the department of obstetrics and gynecology at Dalhousie University in Halifax, Nova Scotia, Canada.
Dodds has studied maternal and fetal risk factors for autism, but she was not involved in the current research.
“I think this meta-analysis was very good,” Dodds says in an email to WebMD.
She says papers like this are very helpful to researchers who are searching for the origins of such a complex disease, but they don't tell the whole story.
“I think it is important to note that because a factor is associated with increased risk, it does not necessarily mean it causes autism,” Dodds says. “These factors may contribute to fetal growth or the overall wellbeing of the fetus and are therefore, possible markers for some underlying, yet still unidentified, cause.”
Kyle Steinman, MD, medical director of the University of Washington's Autism Center in Seattle, says that by pooling data from previous studies, the new analysis clarifies which maternal and fetal risks may be worth paying attention to.
He says the review also helped to identify “methodological differences between prior studies that may account for inconsistencies and that should be considered carefully in designing future epidemiologic studies.”
The study was published in the journal Pediatrics.
Gardener, H. Pediatrics, published online July 11, 2011.
Linda Dodds, PhD, professor and director of research, department of obstetrics and gynecology, Dalhousie University, Halifax, Nova Scotia, Canada.
Kyle Steinman, MD, medical director, University of Washington Autism Center, Seattle.
Hannah Gardener, ScD, University of Miami Miller School of Medicine.
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