Most of us assume we are fastening our children into their safety seats correctly. Still, car crashes are the leading cause of death in children age 4 and up. Car seat mistakes are widespread and dangerous.

1. Buying Used

When you purchase a used car seat, you may not know its full history. Buying new ensures that the car seat hasn't been recalled, is not expired (yes, car seats have an expiration date), has all of its parts, and has never been in car accident. Seats that have been in accidents may be damaged, which will leave your child unprotected if you are in accident.

2. Wrong Seat for Your Child

Child safety seats are each designed with a specific-sized child in mind. Choosing the wrong size can leave your child vulnerable in the event of an accident. Car seat weight limits vary by brand and model, so always make sure you read the recommendations for any seat you consider purchasing.

Usually, but not always, infant seats are for babies that weigh between five pounds and 35 pounds. They must only be used in the rear-facing position. Convertible car seats are usually for children who weigh between five and 65 pounds. They are built to face both the back and front of the car depending on your child's weight and height. Booster seats are only for children between 30 and 100 pounds, and can only be used facing forward.

3. Incorrect Angle

Car seats need to be positioned at the correct angle in order to best protect your child in the event of an accident. The wrong angle could cause life-threatening head and neck injuries to your child. In addition, if your newborn is propped up to high, her head will fall forward, which can make it difficult for her to breathe.

4. Incorrect Buckling

Buckling a child safety seat can be tough. Always read the instructions carefully to ensure that the harness is at the right height for your child. According to the Mayo Clinic, the buckle should fit snug, "allowing no more than one inch of movement from side to side or front to back when grasped at the bottom near the attachment points." The chest clip should be armpit height to ensure that any force from an accident is distributed correctly, rather than on her belly or neck. The straps should lie flat with no slack or twists. Don't dress your child in bulky clothing as it makes it very difficult to correctly buckle them.

5. Incorrect Placement

There has been some confusion lately regarding when to turn your child to face forward. The old rule was age one or 20 pounds. The new recommendation from the American Academy of Pediatrics is age two, because children under two are 75% less likely to die or be injured in a crash if they are in a rear-facing seat. If your car has the Lower Anchors and Tethers for Children (LATCH) system, be sure to follow the directions carefully to ensure your child is protected. Use the tether hook for more stability.

A good tip to follow is to take your installed car seat to a police station that offers free inspections. They will check that your seat is installed correctly for your child, which will give you peace of mind and allow your child safe and comfortable car rides.