In total, 1,862 mothers consented to join the study. Of these, 217 did not meet eligibility criteria (due to maternal and child health, and selective over-sampling of low socio-economic status mothers), and 444 did not participate (e.g. due to refusal).

Lower participation in the study was apparent for mothers recruited from more disadvantaged wards (e.g. as measured by the Child Poverty Index, CPI; Nobel et. al., 2000). The two recruitment sites, North London and Oxfordshire, differed in levels of poverty, with the greater proportion of North London mothers living in relatively disadvantaged areas (below the national average), and a greater proportion of Oxfordshire mothers living in relatively advantaged areas (above the national average). However, when combined, the overall sample had a relatively even distribution of disadvantaged and advantaged families.