A car seat base normally is used with group 0+ infant car seats. It is sold separately from the infant seat, and normally remains permanently fitted in the vehicle - it then allows you to simply click the infant seat on and off the base.
Some group 0+1 and group 1 car seats have a separate base, however at the next stage, seats are normally sold in one unit.
There are two options available for fitting bases - seat belt fitment, or ISOFIX. Some seats will have only ISOFIX or seat belt fitment, but others have both options.
This base may only be fitted with the seat belt.
This base may only be fitted with ISOFIX.
This base may be fitted with ISOFIX or the seat belt.
ISOFIX is considered safer than seat belt fitment, as there are indicators to show you that the base is fitted correctly, when you have a newborn baby this gives you much needed peace of mind!
Infant seats are safe whether they are on a base or not, provided they are compatible to both the car and child; as well as fitted correctly on every journey.
Bases are considered safer as these generally remain fitted in the car, either with the seat belt or using ISOFIX. Your infant seat will click on and off the base, with indicators to show it is fitted properly, which reduces the risk of incorrect fitment and misuse.
Car seat bases experience fitting issues the same as any other car seat, and it is important that you check your base is compatible with your vehicle before you use it. ISOFIX bases will be listed on a compatibility list, but it is also advantageous to visit a retail store in person that has properly trained staff members to check your base and seat fitment.
If your budget allows it, and there is a base available to suit your child seat and your car then it is a good choice to buy a base for the car seat. Not only does it give you peace of mind and makes your life easier with not having to fiddle with seat belts, but it has the added bonus of keeping the seat belt away from the little one's knees as they get older.
Seat belt fitted infant seats normally route the lap belt over the baby's lap, through two blue guides and the chest belt routes around the back of the seat. Whilst this gives a secure fit, it does mean an older baby or toddler may experience the seat belt pressing on their lap.
This is not unsafe or dangerous, however it doesn't look very comfortable and it can be a trigger to move the baby up a seat stage too early.
There is a lot of choices for car seat base but in my opinion, Graco, Britax, and Chicco are the best three of it.