Introduction Epidemiological studies inform understanding of influences on health. Findings can lead directly to policy changes, but such changes need assessing, using formal intervention studies.

Methods The Southampton Women's Survey is a longitudinal birth cohort with data collected on the mothers before conception. 12 583 women aged 20–34 years were assessed when not pregnant; 3159 were then followed through pregnancy and the children are followed-up.

Results Maternal vitamin D levels in pregnancy were positively associated with markers of bone development in the children. Women's educational attainment was strongly related to the quality of their diets before conception, which in turn predicted the quality of the diets of their infants and children. Variations in infant diet were related to body composition at the age of 4 years. Our findings have led to intervention studies. Firstly, we are conducting a randomised controlled trial of vitamin D supplementation in pregnancy (MAVIDOS). Secondly, in relation to our diet quality findings, we are conducting a complex intervention, in collaboration with local policy makers, in which staff working in centres for women and children in disadvantaged areas are trained to engage in ‘healthy conversations’ with young women visiting the centres, to enable them to improve their diets and lifestyles (Southampton Initiative for Health). A school intervention (LifeLab) is also being developed.

Conclusion The Southampton Women's Survey, a large epidemiological study, has led to the development of interventions to improve health of women and their children. These are being evaluated to inform policy, locally, nationally, and internationally.