it is so much easier to change him in that position. as your baby becomes more active you will want to lower the mattress accordingly. once your baby is able to pull up to a standing position put the mattress in the lowest possible position and to be sure your baby is safe, measure the distance between the top of the side bar and the mattress. in the lowest position the distance of the top of the side bar should be no more than 26 inches above the mattress. if your child's head is over the side bar or they climb out of the crib, it maybe time to move 