Breastfeeding is recommended as the best source of nutrition in early life by the WHO. However, mothers who consume large quantities of alcohol while breastfeeding might be harming their offspring because of this ‘passive’ alcohol exposure in the postnatal period. Realistically, in the UK and other developed countries, only a minority of mothers will be consuming alcohol to excess while breastfeeding and caring for a young baby, while most mothers would consume alcohol occasionally and in moderate quantities. These levels of consumption have not been linked with definite harms or benefits to the child so far, with only a few studies examining this question and achieving conflicting results. We propose to investigate the association between early post-natal alcohol exposure and measures of offspring physical and neurological development, including risk of Foetal Alcohol Spectrum Disorder (FASD), in a large UK-based sample representative of the population and followed-up since antenatal appointments, the Avon Longitudinal Study of Parents And Children (ALSPAC).