Babies have weak neck and back muscles that need extra support when traveling in a vehicle. A rear-facing infant seat will support your baby's head and neck in a sudden stop or a crash. Be sure you use a rear-facing infant seat that fits your baby's weight and height.
Rear-facing infant seats are available as infant- only seats, infant/child seats, or infant/child/ booster seats. Infant-only models are rear-facing seats that have a 20-pound weight limit.
An infant/child seat or an infant/child/booster seat can be used as a rear-facing infant seat and later used as forward-facing child seat. Convertible versions work as both rear- and forward-facing seats and can accommodate children who weigh up to 40 pounds.
A forward-facing child seat will spread the forces of a sudden stop or a crash over the strongest parts of your child's body. Use a forward-facing child seat that fits your child's weight and height, when a child outgrows the seat, there may still be a requirement to have them in a child seat appropriate to their new size.
You should replace a forward-facing child seat
Making the transition from a child-safety seat to a booster seat should wait until a child reaches the seat's outer weight and height limits. In other words, if the seat is good for children who weigh 30 to 40 pounds, you should wait until she reaches 40 pounds. When your child weighs 18 kg (40 pounds) or more and typically about 4 1/2 years old, your child is ready to use a booster seat. The booster seat will keep the car's dault seat belt in the correct place over your child's body. You must use the seat belt to hold the booster seat and child in place.
HTSA (in the US) recommends that all children ride in booster seats until they're at least 8 years old or 4 feet 9 inches tall or over 27 kg (60 pounds). At this point they are tall enough that the shoulder belt hits mid-chest and shoulder (and not at the neck or throat), and the lap portion of the belt is low across the thighs (and not the stomach). Children should be tall enough to sit against the backrest with their legs bent at the knees and feet hanging down.
Many children have been hurt or killed in a crash because they were too small to use a seat belt, which is why special seats were introduced. The back seat is always the safest place for your child.
Transport Canada Guidlines for Child Seats