['A relation between birth weight and adult body composition has been reported in singleton populations, especially when more accurate measures of body composition, such as dual-energy X-ray absorptiometry (DXA) were used. It remains uncertain whether this is mediated by a direct effect of fetal nutrition, through factors in the shared environment, or through genetic factors.', 'The objective was to investigate the relation between birth weight and body composition with the use of a co-twin design.', 'DXA measurements and birth weights were available for 2228 dizygotic and 842 monozygotic female twins aged between 18 and 80 y. Multivariate regression models were used to identify both individual specific relations and those mediated through the shared environment.', 'Significant relations were found between birth weight and DXA measures for individuals. A 1-kg increase in birth weight was associated with a 1.72-kg increase in lean mass, a 0.25-kg increase in fat mass, and a 0.05-unit increase in the lean:fat mass ratio. Within twin pairs, the analysis showed that associations between birth weight and absolute levels of lean and fat mass were mediated through individual-specific effects, whereas the relation between birth weight and the proportion of lean to fat mass was mediated purely through factors common to twin pairs.'] Is an obesogenic postnatal environment more important than the fetal environment for the development of adult adiposity : a study of female twins?