Low birth weight has been classified as a risk factor for cardiovascular disease by the World Health Organization (WHO).1 This strategy was supported by many observational studies since 1980s, showing lower birth weight was associated with higher blood pressure in later life.2-4 Some researchers explained this inverse association as a 'reverse paradox', which resulted from an over-adjustment for current weight status.5 A recent study of the 1958 British birth cohort used twin status as an instrumental variable, and reported no effect of birth weight on hypertension.6 Nevertheless, it remains unclear that to what extent the effects of birth weight on blood pressure are mediated by body mass index (BMI) throughout life.