Before buying, read the labels on the car seat or booster seat. Look for weight, height, and age limits to make sure the car seat is right for your child. Make the last stop on the car seat journey stress-free by confidently choosing what's right for you and your child. Once the forward-facing car seat with a harness is too small for your child, it's time to travel in a booster seat, but still in the back seat.
For example, the Maxi Cosi Magellan convertible car seat can support up to 40 pounds facing backwards, but the height limit is 40, which is likely to happen before your child reaches 40 pounds. You could be gradually leaving a seating position, the LATCH, the 5-point harness, or a full car seat without even realizing it. The convertible car seat, as the name suggests, can be converted from a rear-facing car seat to a forward-facing car seat. These recommendations are summarized in the following image, which is extracted from the NHTSA children's car safety guide.
If you're going to use an infant car seat, you're probably doing it because it adds the convenience of being able to take the car seat out of the vehicle and place it directly in a stroller. As you can see in the chart, your baby will always start in the rear-facing position and maintain that position until they reach the height or weight limit of the car seat manufacturer. Never buy a used car seat at garage sales, flea markets, thrift stores, or online if you don't know the previous owner or the full history of the car seat. Most children will never reach the high weight limits of modern infant and convertible car seats before reaching the height limit.
If the top of your child's head is less than an inch from the top of the car seat shell, it's time to upgrade it. Some of the larger infant car seats and convertible seats can be very large in a smaller vehicle, and they even push up against the front passenger seat when installed facing rearward. But they are also restricted in a number of ways, including their inability to position themselves in relatively upright positions rearward (for young children), their limited legroom against the back seat, and their limited weight and height restrictions. The safest and best car seat is the one that installs securely in your vehicle, that adapts to your child's age, height, and weight, and can operate properly at all times.
A used car seat isn't safe if you've been in an accident or if you're missing parts, labels, or instructions. Traditionally, this was limited by the car seat manufacturer's low limits for rear-facing weight and height limits.