The recommendation is that the backrest should be approximately midway between vertical and horizontal. If the backrest is too upright, especially in the case of small children, their heads may fall sideways or on the chest, which can make it difficult for them to breathe. There are child seats that can be tilted between 30º and 45º from the vertical. The youngest babies will start at 45º and, as they grow, the seat will need to be adjusted to 30º. You should always check the child seat’s instruction manual to find out the correct inclination in each case.