In November 2012, a large study suggesting that flu during pregnancy raises autism risk in the child captured headlines in the popular press. In reality, the risk is small, increasing from an average of 0.4 percent to 0.9 percent. (Even the study’s author called the coverage “out of proportion.”)
A larger study, published in January in PLoS ONE, suggests that other health issues during pregnancy also have little impact on autism risk.
The more serious risk, the study found, is for intellectual disability, which can often occur along with autism.
Previous research on maternal health and autism has provided mixed results, but a number of studies have linked low birth weight, premature birth and slow fetal growth rate to risk of the disorder. The new study is one of the largest to look at the link between maternal, as well as fetal and newborn, health, and autism with or without intellectual disability.
Researchers analyzed data on all live, single births in Western Australia between January 1984 and December 1999, a total of 383,153 births. They then looked for links between maternal health factors — such as high blood pressure — as well as labor and delivery, newborn health and socioeconomic factors, and autism. (They did not look specifically at maternal flu, though they did look at urinary tract infections.)
According to the findings, autism with and without intellectual disability have different risk profiles.
No maternal health conditions or perinatal factors — which relate to the few weeks before and after birth — are linked to increased risk of autism in the absence of intellectual disability.
Poor fetal growth and threatened abortion, or bleeding in the first 20 weeks of pregnancy, are linked to an increased risk of autism with intellectual disability.
Intriguingly, high blood pressure in the mother and small head circumference in the child at birth are linked to a slightly reduced risk of autism.
The reasons for this finding are unclear: Maternal hypertension and small head size have both previously been linked to an increase in autism risk.
The researchers speculate that the hypertension finding might be a fluke, or that magnesium sulfate, which some pregnant women take to control blood pressure, has a protective effect against autism. The drug has been shown to reduce risk of cerebral palsy, another neurodevelopmental disorder.
The mother’s health has a much stronger influence on intellectual disability, however. Diabetes, high blood pressure and asthma in the mother are all linked to intellectual disability in the child, as is premature labor.
The findings suggest that when looking at risk factors, researchers should separate autism and intellectual disability, rather than consider them together.