Weight-for-age is an index comparing a child’s weight with the average weight of healthy children the same age — expressed as a percentage or a z-score. It is a composite indicator: a low weight-for-age can mean the child is thin now (wasting), short from long deprivation (stunting), or both. Health workers use it to screen for undernutrition in growth-monitoring programmes, plotting measurements on growth charts at clinic visits. Its weakness is its bluntness — it cannot distinguish acute from chronic problems — which is why it is used alongside weight-for-height and height-for-age. A falling weight-for-age curve is an early warning that a child is not thriving and needs assessment.