A growth chart is used by pediatricians and other health care providers to follow a child's growth over time. Growth charts have been constructed by observing the growth of large numbers of healthy children over time. The height, weight, and head circumference of a child can be compared to the expected parameters of children of the same age and sex to determine whether the child is growing appropriately. Growth charts can also be used to predict the expected adult height and weight of a child because, in general, children maintain a fairly constant growth curve. When a child deviates from his or her previously established growth curve, investigation into the cause is generally warranted. Parameters used to analyze growth charts include weight velocity (defined as rate of change in weight over time), height velocity (defined as rate of change in stature over time), and whether someone's growth chart crosses percentiles. For instance, endocrine disorders can be associated with a decrease in height velocity and preserved weight velocity while normal growth variants are associated with a decrease in height and weight velocity that are proportional to each other. It's important to note that other parameters are more commonly used such as waist circumference for assessing obesity and skin fold difference for assessing malnutrition. Growth charts can also be compiled with a portion of the population deemed to have been raised in more or less ideal environments, such as nutrition that conforms to pediatric guidelines, and no maternal smoking. Charts from these sources end up with slightly taller but thinner averages. Growth charts are different for persons assigned male at birth and female at birth, due in part to pubertal differences and disparity in final adult height. In addition, children born prematurely and children with chromosomal abnormalities such as Down syndrome and Turner syndrome follow distinct growth curves which deviate significantly from children without these conditions. As such, growth charts have been created to describe the expected growth patterns of several developmental conditions. Since there are differences in normal growth rates between breastfed and formula-fed babies, the World Health Organization growth charts, which better reflect the growth pattern of the healthy, breastfed infant are considered the standard for U.S. children under age two. History and Revisions to Growth Chart The growth chart was first developed by the National Center for Health Statistics (NCHS) in 1977 to clinically analyze child development. The 1977