BIS is a population-derived birth cohort study of more than 1,000 mother-infant pairs from the Barwon region in the south-east of Australia. Between 2010 and 2013, the study recruited pregnant mothers who were at no more than 28 weeks of gestation and were planning to give birth at either Geelong Hospital (public) or St John of God Hospital (private). The mothers and their offspring (including 10 twin pairs) were assessed at birth and are being continuously followed up at multiple stages across development.