Swaddling is when babies are wrapped up tightly in a blanket with just the head exposed.
According to the study, swaddled babies placed on their sides or stomachs are twice as likely to die from the syndrome. Researchers say overheating could be a factor.
They also suggest that parents who do swaddle their babies pay attention to their position.
The risk is less for babies who sleep on their backs.
Rolling to the side or stomach may hinder a child's breathing.