Between April 25, 2009, and March 25, 2010, all pregnant women in three areas in New Zealand were eligible for recruitment in the GUiNZ, resulting in over 6,000 consenting mothers' participation over 21 years. The cohort comprises over 6,000 children, allowing for robust analyses of developmental trajectories, particularly among subgroups like Māori, Pacific, and Asian children, each with at least 1,000 representatives. Demographic characteristics of the recruited cohort mirror those of New Zealand's parent population, including maternal age, ethnicity, parity, and area-level deprivation, with an increasing proportion of children born to mothers born elsewhere.