Think of your child’s car seat as a parachute that slows her fall and cushions her landing in a crash. If you were to jump from a third-story window (which is the equivalent impact of a 30-mile-an-hour crash) a parachute that is snugly attached to your body will bring you to a stop that is as slow and gentle as possible. The same applies to a harness, which should fit very snugly to your child’s body, but rarely does, because most parents worry about their child’s comfort when they snap them in. “Your parenting instinct tells you it will be better loose,” says Dr. Baer. But making it snug is a much safer choice, and not uncomfortable.

The fix:

1. The harness should be snug enough “that you can only fit one finger between your child’s collarbone and the harness strap,” says Emily Levine, a nationally certified child passenger safety technician and one of theCar Seat Ladies.

2. The chest clip should be even with the armpits.

3. Children should not be wearing bulky clothes like jackets when they ride in their seat.

4. Do not use any accessories that are not certified for use in car seats, like bundlers, car seat covers that did not come with the seat, or head positioners. “The only thing you should add to a car seat is a child,” says Levine.