An epidemic of childhood obesity has occurred in recent years, beginning in the late 1980s in the United Kingdom.1 It is of concern because of adverse consequences in the short term and long term.1 2 The identification of risk factors is the key to prevention.3 Evidence on risk factors for childhood obesity is limited at present,4 5 although awareness is increasing for the importance of the environment in early life. Almost all recognised risk factors are potential rather than confirmed. A systematic review found that most previous studies on risk factors for obesity were unable to adequately account for confounding variables (particularly socioeconomic status),5 were mainly cross sectional, were underpowered,4 5 and failed to investigate the effect of several potential risk factors simultaneously.