One of the challenges of looking for the causes of autism is that some findings are misleading. Just because something is related to autism doesn't it mean it caused it. Such is the dilemma in the findings of a recent study looking at autism risk and babies born early.
Researchers found that the earlier babies were born before they were full term, the more likely it was that they were later diagnosed with autism.
However, it is not clear that being born early caused the developmental disorder.
There may be other underlying factors related to both autism and preterm birth that scientists need to learn more about.
This study, led by Michael W. Kuzniewicz, MD, MPH, of the Division of Research at Kaiser Permanente Northern California, aimed to find out whether autism risk varies according to the week of pregnancy when a child is born.
The researchers examined the cases of 195,021 babies born at 24 weeks of pregnancy or later at one of 11 Kaiser Permanente Northern California hospitals between 2000 and 2007.
The researchers then looked to see how many of these children had been diagnosed with an autism spectrum disorder at a Kaiser Permanente center, by a specialist or by a pediatrician.
The researchers found that the group of babies born early had slightly higher rates of autism than the babies born on time or later. A full-term pregnancy is regarded as being between 37 and 42 weeks.
Among babies born before 37 weeks of pregnancy, 1.78 percent were diagnosed with autism.
Among babies born at 37 weeks of pregnancy or later, 1.22 percent were diagnosed with autism later.
As the researchers broke down the categories of birth week further, they found that the earlier a child was born, the more likely a later autism diagnosis was.
Babies born between 24 and 26 weeks of pregnancy were 2.7 times more likely to be diagnosed with autism than babies born at full term.
Babies born between 27 and 33 weeks of pregnancy were 1.4 times more likely to be diagnosed with autism than babies born at full term.
Those who were born only slightly preterm, between 34 and 36 weeks of pregnancy, were 1.3 times more likely to be later diagnosed with autism.
When the researchers looked at birth complications as well, they found that the risk of being diagnosed with autism was higher for preterm babies born before 34 weeks of pregnancy who experienced one of two complications.
One complication was intracranial hemorrhage, or bleeding in the baby's skull.
The other complication was a baby's need for high frequency ventilation, or artificial breathing with a machine set to help babies breathe at four times the normal rate of breathing.
"Each week of shorter gestation was associated with an increased risk of autism spectrum disorders," the researchers wrote.
However, these findings do not mean that being born early causes children to develop autism. It could be that the underlying cause of autism is part of a condition that actually leads to the preterm birth instead.
Or there could be some other factor at work in the link between the two.
"Conditions leading to poor intrauterine growth [growth in the womb] such as placental insufficiency [not enough nutrition from the placenta] may contribute directly to an increased risk of autism spectrum disorders or indirectly through other associated conditions such as intracranial hemorrhage," the authors wrote.
Glen Elliott, MD, PhD, a clinical professor at the Stanford University Department of Psychiatry and Behavioral Sciences, noted that clinicians have been concerned for a long time that premature babies might be at an increased risk for autism just as they are at higher risk for other psychiatric disorders.
"These investigators have taken nice advantage of a large sample available through a single health care system to quantify that risk," he said of this study.
"It is important information that may help in the ongoing search for causes of autism, but it is important to emphasize that, although the increased risk is sizeable, that absolute risk still remains relatively low even for the most at-risk group," he said.
This study was published October 22 in The Journal of Pediatrics. The research was funded by the Kaiser Permanente Northern California Community Benefit Program. The authors declared no conflicts of interest.