From newborn and up until the child's weight or height reaches the maximum weight or height limit according to the car seat instructions for infant and convertible car seats. Keep your child rear-facing longer. With convertible car seats (rear-facing then forward-facing) going to a higher weight, some allow children to ride rear-facing up until 40 and even more car seats up to 50 pounds. In these seats a child can ride rear-facing to around 3-5 years old. This significantly reduces the risk of c-spine (neck) and brain injuries from riding forward-facing too soon.
The infant carrier is used where the base is installed in the vehicle rear-facing and the carrier (which has the baby fastened with the 5-point harness) clicks into the base. This is recommended for newborns and use up to the maximum of the height or weight recommendations of the seat. In most cases the baby outgrows the height and becomes too heavy for the adult to carry the baby in the carrier. When the baby is small the carrier is most convenient for those with more than one vehicle where a base can be in each car. This is helpful for when you have to make numerous stops while driving. It makes it easier to carry the baby form one place to another without disturbing them and has the ability to use with a stroller.
Convertible Seat (rear-facing & forward-facing)
The convertible seat is installed directly into the vehicle. The baby then placed into the seat, then fastened with the 5-point harness. These seats can be installed rear-facing, then once the baby reaches the maximum height (which is usually the case first) or weight maximum of the seat it can then be re-installed forward facing. The all-in-one seat is similar however has an additional booster seat stage.