June 2, 2008 -- Low birth weight and preterm birth increase the risk of autism in infants by about twofold, but more so for girls than for boys, according to a new study.
The research bolsters the suspected link between autism, low birth weight, and prematurity.
"We saw this difference in risk between boys and girls," says Diana Schendel, PhD, lead health scientist at the National Center on Birth Defects and Developmental Disabilities at the CDC in Atlanta. The gender finding was unexpected, she says, especially since autism is much more common in boys. Schendel is a researcher of the study, published in Pediatrics.
When the 565 boys and girls with autism were looked at separately, the boys had less than a twofold increased risk of autism if they were born at low birth weight, but the low-birth-weight girls had a threefold or higher risk, found Schendel and her CDC colleague Tanya Karapurkar Bhasin, MPH.
They also found that low birth weight (less than 5.5 pounds) and early preterm birth (less than 33 weeks' gestation) affected groups of children differently, depending on whether they had autism alone or autism and other developmental disabilities.
"There may be a lot of variation in the endpoint we call autism," Schendel tells WebMD. The study result, she says, "really is highlighting that we aren't looking for one cause of autism." The study builds on previous research, some of which has also found a link between low birth weight and autism.
"What is new in this study is the in-depth look at the gender effect," she says. The analyses of different groups classified by number and type of disability also add new information.
Study Details: Autism and Low Birth Weight
The researchers examined the records of 565 children with autism, born in Atlanta from 1986 to 1993, matching them to a control group of children. They looked at whether the children were born early, small, or both, and whether they had autism and other developmental problems.
The children with autism were further divided into three groups: those who had autism but no other developmental disabilities, those with autism who also had an intellectual disability, and those with autism and more than one other developmental disability.
About one in 150 children in the U.S. now have autism or related disorders that fall on what is known as the autism spectrum, according to the CDC.
Overall, the researchers found that a birth weight below 5.5 pounds was associated with a 2.3-fold increased risk for autism. Although preterm birth earlier than 33 weeks was not statistically linked to autism for boys, there was a significant fivefold increased risk seen for girls with autism.
The researchers then looked at the subgroups, divided by type and number of disability, and found variations. For instance, the increase in risk for autism in low-birth-weight girls with mental retardation was fourfold, while the increased risk found in low-birth-weight boys just for developing autism was not statistically significant.
Schendel can't explain why the low-birth-weight girls were found to have a greater risk than the low-birth-weight boys. Both low birth weight and preterm birth are markers, she says, that something may have gone wrong during the pregnancy. The poor fetal growth resulting in low birth weight may be associated with developmental problems. Or, on the other hand, low birth weight may be a marker of a fetus that's already adversely affected neurologically, she says.
Second Opinion: Low Birth Weight and Autism
The study is scientifically solid, says Susan Hyman, MD, an autism researcher and an associate professor of pediatrics at the University of Rochester Medical Center, University of Rochester School of Medicine, N.Y. She chairs the autism expert panel of the American Academy of Pediatrics.
Though previous research has yielded conflicting findings about the low birth weight-autism link, "I think this study comes close to laying the debate to rest," Hyman says.
"Pediatricians are very aware to monitor babies [born prematurely or at low birth weight] that require neonatal intensive care unit care for any sort of developmental difficulties," she says. But autism has not been targeted specifically; this study may make them more aware, she says.
Under the academy's developmental surveillance guidelines, Hyman says, children born early and small should be monitored for developmental disabilities. And the academy recommends routine screening for autism at 18, 24, and 30 months and also when there are parental concerns, she says.
Advice for Future Parents
While there's no "cure" to avoid low birth weight, Schendel advises pregnant women to seek prenatal care as early as possible.
"Discuss with your doctor your lifestyle," she says. It's important, for instance, for pregnant women not to smoke cigarettes and to avoid infections, among other healthy lifestyle habits.
Hyman adds: "Don't smoke, don't drink. Take your vitamins. Eat well. Sleep enough. Moderate exercise [as recommended by your doctor] is good. And don't worry. Stress is bad for pregnant women."