Birth weight remains a major focus of medical research into the relationship between pre-natal growth and life course health, and historians have used mean birth weight to assess women's standard of living. However, there are intrinsic difficulties in inferring maternal health and nutritional status from birth weight, and some of the known data sets produce puzzling results. One rich data set comes from the Melbourne Lying-in Hospital, 1857-83, and the article discusses the complex institutional, social, and economic causes that may underlie its apparently counter-intuitive anthropometric results. This data set reveals the biological effects differential social conditions can inflict, even within an otherwise affluent society.