Development is a continuous process that begins at conception and proceeds stage by stage in an orderly sequence. There are specific milestones in each of the six areas of development (cognitive skills, gross motor skills, fine motor skills, language skills, social development and self-help skills) that serve as prerequisites for the stages that follow. Most children are expected to achieve each milestone at a designated time, also referred to as a "key age," which can be calculated in terms of weeks, months or years. Because of specific challenges associated with Down syndrome, babies will likely experience delays in certain areas of development. However, they will achieve all of the same milestones as other children, just on their own timetable. In monitoring the development of a child with Down syndrome, it is more useful to look at the sequence of milestones achieved, rather than the age at which the milestone is reached.