Every 18 minutes in the U.S., a baby who will acquire autism is born. Despite its widespread prevalence, scientists do not know what causes the developmental disorder—an array of genetic and environmental factors are probably involved. One such variable, a new study suggests, might be the womb: mothers of autistic children may produce immune proteins that react with and potentially harm their babies’ brains during pregnancy.

Past studies have linked autism to the immune system—especially to autoimmune reactions, in which the body’s defenses mistakenly attack native tissue. Autistic people are more likely than healthy subjects to make antibodies against their own brain cells, and autoimmune disorders such as type 1 diabetes are more common in mothers of autistic kids. Harvey Singer, a pediatric neurologist at Johns Hopkins University, wondered whether mothers of autistics might have passed aberrant antibodies to their children during pregnancy.

Singer and his colleagues collected blood samples from 100 mothers of children with moderate to severe autism and 100 mothers of healthy children. They extracted only the antibodies that could cross the placenta during pregnancy, then tested these antibodies against proteins from human fetal brain tissue. The team found that the blood from the mothers of autistic children reacted more strongly than that of the mothers of normal children against at least two fetal brain proteins. The two groups of mothers had reactions similar to each other against the other proteins.

“These immune factors may help turn on or trigger some potential underlying problem,” Singer speculates. He does not yet know, however, the role the brain proteins play during development or whether the maternal antibodies actually influence their function. The team plans to investigate these questions by injecting human maternal antibodies into pregnant mice to see if their offspring show developmental problems.