MANY known risks for autism occur during late pregnancy and birth. Premature birth is a risk for developmental disability, including autism. Notably, elective cesarean section is associated with an autism risk ratio of 1.9. Since a substantial proportion of early deliveries are elective, without a compelling medical reason, this risk is preventable.

A highly underappreciated prenatal risk is stress. For pregnant women who take the sometimes-wrenching step of emigrating to a new country, for example, the risk ratio is 2.3. In the fifth through ninth months of pregnancy, getting caught in a hurricane strike zone carries a risk ratio of about 3. Maternal post-traumatic stress disorder during pregnancy is associated with a similar effect. These events are likely to trigger the secretion of stress hormones, which can enter the fetus’s bloodstream and affect the developing brain for a lifetime. Stressors may also lead to maternal illness, the immune response to which may interfere with brain development.

Stress might account for other findings as well. Recent news coverage has speculated on the influence of air pollutants, which carry risk ratios around 1.4. This risk might be caused by chemicals — or by the stress of living in a poor or crowded neighborhood, where pollution is worse. A larger risk comes from households that already have an older sibling under 1 year of age, where newly conceived children have a risk ratio for autism of 3.4. So sure, parents should avoid smog — but also might think about spacing their children at judiciously chosen intervals.

After birth, known risks diminish. But the baby’s brain acquires a new need: social experience. In one group of Romanian orphanage children, babies were nearly isolated from social contact, and some later showed autism-like symptoms. Developing brains go through sensitive periods during which they require a minimum level of normal experience. Extreme deprivation may affect a critical period of the brain’s social and emotional development.

Risk ratios are good not just for parents, but also for researchers, who can follow them to new areas for in-depth study. The statistics suggest a conceptual framework in which fetal brain development stays on track unless it is driven awry by genetic hits, by adversity in late pregnancy, or a combination of the two.

In my laboratory, risk ratios have led us to examine particular brain regions. Many brain regions in autistic people show abnormalities, but it is not known whether some malfunctioning regions cause other regions to go off track. In one study focusing on detailed long-term outcomes in 51 children, damage to the cerebellum at birth leads to a risk ratio of about 40. The cerebellum links information arriving from different senses and communicates with nearly all regions of the cerebral cortex. Many known risk genes for autism are turned on together in the cerebellum in early life. Although cerebellum injury is rare, we think the cerebellum might be important in using sensory experience to guide normal brain development.