Baseline characteristics
In table 1, we summarise the baseline characteristics of participants in the study by breastfeeding groups. Among the 43 383 infants, 54% were breast fed for at least 6 or 7 months: 22% were exclusively breast fed and 32% were partially breast fed. Only 6% were exclusively formula fed. Those in the exclusively breastfed group were born heavier and with more older siblings on average. Examination of maternal age at delivery and socioeconomic factors demonstrated that those in the exclusively breastfed group and partially breast fed for at least 6–7 months group were nurtured in a more socially endowed environment (eg, higher family income, more educated mothers, non-smoking mothers or mothers older than 25).