Winter Car Seat Safety Tips from the American Academy of Pediatrics (AAP)

Nothing bulky should ever go underneath your child's body or between her body and the harness straps. Be sure to leave your baby's face uncovered to avoid trapped air and suffocation.
As a general rule, bulky clothing, including winter coats and snowsuits, should not be worn underneath the harness of a car seat.

In a car crash, fluffy padding in a coat immediately flattens out from the force, leaving extra space under the harness. A child can then slip through the straps and be thrown from the seat.

Remember, if the item did not come with the car seat, it has not been crash tested and may interfere with the protection provided in a crash. Never use sleeping bag inserts or other stroller accessories in the car seat.