The study determined that exposure to a type of pesticide commonly used on crops eaten by U.S. consumers could be having this effect.
The pesticides are known as organophosphates. They kill insects by disrupting their brains and nervous systems.
The effects seen on babies in the study were relatively small. But experts say, spread out over millions of babies, lighter babies and shorter pregnancies could have serious health consequences.
The study is in the journal Environmental Health Perspectives.