One of the basics of correct car seat installation, in addition to buckling the car seat tightly into your vehicle, is that your child is 'buckled snugly' into the car seat itself. If the harness straps are loose, then your child can be injured or could even fly out of the car seat if you are in a crash.
Keep in mind that thick and heavy winter coats, which may become compressed under the harness straps in a crash, become too loose, and may allow your child to become injured or even ejected from the car seat and/or car.
Instead of putting your child in a car seat with a heavy winter coat, it is recommended that you keep your child in the clothes they will be wearing when they are indoors. Place the child in the infant car seat, making sure the harness straps are snug at the collar bone, is at or below the shoulders, that they lie straight and flat down to the buckle. Ensure that the chest clip is at arm pit level. Buckle the child in and then put the coat or blanket over them -- on top of the harness system. You can even turn the coat around and put their arms through the sleeves after they are buckled into the car seat. This will ensure your kids are kept snug in their car seat, the harness system is able to do its job in the event of a crash, and that the kids stay warm.
The NHTSA also recommends that to keep your baby the safest, always remove bulky clothing or blankets before you place the child in the car seat. Then, put the blanket or coat over the baby. You should never place anything thick underneath the baby, unless that item came with the car seat originally --which tells you it's been tested by the manufacturer.
When a child is wearing a thick coat, it's hard to tell if you have a good harness fit, which is crucial. A coat can add a lot of slack, reducing the level of protection for your child in a crash.
Courtesy of American Academy of Pediatrics. Car Safety Seats: A Guidefor Families 2006 By Vincent Iannelli, M.D., About.com Guide
Thanks to www.childcarseatsafety.ca for sharing this article with us