Background: Weighing breastfed babies has been the subject of some controversy as the previous international growth chart was largely based on data from infants fed infant formula. The concern that professionals may be misled by the charts into suggesting to mothers that they supplement unnecessarily was a major impetus for the World Health Organization's investment in a new growth chart. Evidence of interpretation in practice has been scant. Methods: An ethnographic study was conducted in a town in the Northwest of England to investigate this issue. In the first phase, women and health visitors were observed in the well-child clinic during clinic sessions and breastfeeding group meetings. In the second phase, longitudinal interviews with 14 women were conducted. Each woman was interviewed up to three times in the first six months after the birth of her baby, with a total of 35 interviews. Results: Mothers and health visitors focussed on weight gain with frequent weighing and attention to even minor fluctuations of the plotted line being evident. Women felt it important to ensure their baby's weight followed a centile, and preferred for this to be the fiftieth centile. Interventions included giving infant formula and solids as well as changing what the mother ate and drank. Women also described how they worried about their baby's weight. Little effective support by health professionals with breastfeeding technique was observed. Conclusion: Babies were weighed more often than officially recommended, with weighing and plotting being at the core of each clinic visit. The plotted weight chart exerted a powerful influence on both women's and health visitors' understanding of the adequacy of breastfeeding. They appeared to rate the regular progression of weight gains along the chart centiles more highly than continued or exclusive breastfeeding. Thus weighing and visual charting of weight constituted a form of surveillance under the medical gaze, with mothers actively participating in self monitoring of their babies. Interventions, by mothers and health visitors, were targeted towards increasing weight gain rather than improving breastfeeding effectiveness. Improvements in training are needed for health visitors in weighing techniques, assessing growth patterns - particularly of breastfed babies - and in giving information to women, if the practice of routine weight monitoring is to support rather than undermine breastfeeding.