Birthweight is one of the most accessible and most misunderstood variables in epidemiology. A baby's weight at birth is strongly associated with mortality risk during the first year and, to a lesser degree, with developmental problems in childhood and the risk of various diseases in adulthood. Epidemiological analyses often regard birthweight as on the causal pathway to these health outcomes. Under this assumption of causality, birthweight is used to explain variations in infant mortality and later morbidity, and is also used as an intermediate health endpoint in itself. Evidence presented here suggests the link between birthweight and health outcomes may not be causal. Methods of analysis that assume causality are unreliable at best, and biased at worst. The category of 'low birthweight' in particular is uninformative and seldom justified. The main utility of the birthweight distribution is to provide an estimate of the proportion of small preterm births in a population (although even this requires special analytical methods). While the ordinary approaches to birthweight are not well grounded, the links between birthweight and a range of health outcomes may nonetheless reflect the workings of biological mechanisms with implications for human health.