Phase 1 car seats are infant seats also known as "shell". They are rear-facing seats only with a base.
These models are intended for babies. Their backrest is therefore tilted to provide good support for the neck and head. In the event of a sudden stop or a head-on collision, your child's head cannot be thrown forward because it is supported by the back of the seat.
As soon as your child is born, it is important to get a shell (which only covers phase 1). Because it will be required as a condition to authorize your discharge from the hospital after the birth according to the chosen maternity / birth center.
Although many "convertible" "convertible" seats cover phase 1, they are not admitted on discharge conditions to authorize your discharge from the hospital after the birth depending on the maternity / birth center chosen. The latter have the particularity of being fixed in the car and are not "mobile".