Adjustable mattress heights are a common feature of most cribs. The ability to raise and lower the mattress can make putting your child to bed and getting him out of bed easier. While this feature does have many advantages, leaving the mattress too high can increase the risk of a child injuring himself. The correct height for a crib mattress depends on your child's age and abilities.
Very young babies can sleep in a crib with the mattress set to the highest level. Between birth and approximately five months, developmentally normal babies are not capable of sitting up without support or pulling themselves into a sitting position. At this stage, there is little risk that the child can pull himself up and out of the crib. According to the American Academy of Pediatrics, keeping the crib mattress high if the child is in this age range and not able to sit up is safe.
Once a child can sit up, the crib mattress should be lowered down one notch or halfway, depending on the type of crib you have. This stage occurs anywhere between 5 and 8 months old, depending on the development of the child; it can, however, occur much earlier or later. A child who can sit up is more likely to attempt to pull herself up, especially when no one is looking; many parents have found their child standing for the first time when going into her room to get her. Once the child can stand, there is a chance she can pull herself up enough to climb out of the crib.
As soon as a child shows signs of pulling herself up to a standing position, the crib mattress should be adjusted to its lowest setting. This height will deter most children from attempting to climb out of their crib, although it is still a possibility. To make using her crib to practice climbing more difficult, remove any bumpers or padding that could possibly be used as steps.
Moving on From the Crib
Once a child is over 35 inches in height, you need to move him to a toddler bed. At this point, the crib mattress can no longer be adjusted low enough to ensure the safety of the child, and the height of the crib rails coupled with the child's increased gross motor skills makes successfully climbing out of the crib much more likely. This can result in injuries not only from a fall but can potentially leave the child unsupervised in the room if he's quiet.