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A number of observational cohort studies, including some from the UK, have shown lower rates of long-term obesity in children who are breastfed for longer. This is one of the many potential benefits used to encourage hesitant new mothers. However all these studies are inevitably biassed by confounding: whatever it is that makes a mother choose to breastfeed is associated with other, sometimes unmeasurable, favourable parenting practices. Thus only a randomised trial could show any true benefit.