Getting The Harness Right
When a child sits in their car seat, the only thing holding them in place is the seat's five-point harness, so it is vital that that harness is correctly fitted and adjusted.
Correct Shoulder Strap Height
In a rear facing car seat the shoulder straps should be level with, or just below the child's shoulders. This is especially important when the seat is reclined. It is the harness's job to hold the child in the seat and if the shoulder straps are too high, the child will slide up the seat's back rest in a crash subjecting the head and body to unnecessarily high forces. Having the straps above the shoulders can be just as dangerous as the harness being too loose.
Make Sure It's Tight
A car seat's harness will only be effective in a crash if it is tightly adjusted across the child's body. A loose harness allows unnecessary movement in a crash which causes injuries, it enables the child to wriggle their arms out of the straps, subjecting it to head and neck injuries, and it can cause the child to be ejected from the seat altogether.
Get into the habit of loosening the straps before taking your child out of the seat. That way you will have to adjust them again every time you put him back in, and you can make sure that they are always tight enough.
After doing up the buckle, pull the shoulder straps up so that the harness lies flat on the child's hips. Then pull the adjuster strap at the bottom of the seat to tighten the shoulder straps. Make sure there are no twists in the harness. Don't worry about hurting your baby by 'over-tightening' the harness, this is virtually impossible. Finally pull down the chest pads, these are designed to stop the straps from sliding off the shoulders.
You can check that the harness is tight enough by doing the 'pinch-test'. Once the harness has been fastened and tightened, try to pinch some of the harness strap webbing between your index finger and thumb. Your fingers should slide off the harness, if you are able to pinch the webbing between your fingers the harness is too loose and needs to be tightened more.