1. Buying the wrong seat for a particular carSome seats fit better in certain cars than others. But the only way you'll know which seats work best in your car(s) is through trial and error. If possible, months before your baby is born, go to a baby store and ask to install several different models in your car until you find one that fits. "I tell people to do this before they start looking at cribs," says Prom. "A safe car seat is one of the most important purchases a parent can make for her baby."
2. Facing baby forward too soon and/or using a seat that doesn't fit properlyInfants should be in the rear-facing position, in either an infant carrier (a seat with a carrying handle) or a convertible seat (a seat you can turn forward when your baby is big enough) until they are one year old and weigh at least 20 pounds, whichever comes last. If your under-one-year infant is so tall that her head reaches the top of the infant carrier, you need to switch her into a convertible seat but keep her facing rear. Many convertibles accommodate children in the rear or forward position until they weigh about 35-40 pounds.
3. Not tightening the car seat enoughMake sure the seat doesn't move more than approximately one inch from side to side or front to back. Read the car seat manufacturer's instructions so you know where to thread the seat belt, and your vehicle's manual so you know whether you must use a locking clip to secure the seat belt. Be sure to install the locking clip next to the latch plate.
4. Positioning the harness height incorrectlyRead the car seat's instruction manual to determine the proper harness height. Some harnesses should sit level with your infant's shoulders, others should be set just below. You will need to adjust the harness height as your baby grows.
5. Not tightening the harness enoughAdjust the harness so you can't slide more than two fingers between the harness and your baby. And make sure that the chest clip is at your baby's armpit level to keep the harness in place.
"This is one time it's essential to read instructions," says Julie Prom, a certified child passenger safety expert based in Stafford, Virginia. "Read the car seat manufacturer's guide and your auto manual before you install a car seat."