BACKGROUND: Investment in strategies to promote 'a healthy start to life' has been identified as having the greatest potential to reduce health inequalities across the life course. The aim of this study was to examine social determinants of low birthweight in an Australian population-based birth cohort and consider implications for health policy and health care systems. METHODS: Population-based survey distributed by hospitals and home birth practitioners to >8000 women six months after childbirth in two states of Australia. Participants were women who gave birth to a liveborn infant in Victoria and South Australia in September/October 2007. Main outcome measures included stressful life events and social health issues, perceived discrimination in health care settings, infant birthweight. RESULTS: 4,366/8468 (52%) of eligible women returned completed surveys. Two-thirds (2912/4352) reported one or more stressful life events or social health issues during pregnancy. Women reporting three or more social health issues (18%, 768/4352) were significantly more likely to have a low birthweight infant (< 2500 grams) after controlling for smoking and other socio-demographic covariates (Adj OR = 1.77, 95% CI 1.1-2.8). Mothers born overseas in non-English speaking countries also had a higher risk of having a low birthweight infant (Adj OR = 1.85, 95% CI 1.2-2.9). Women reporting three or more stressful life events/social health issues were more likely to attend antenatal care later in pregnancy (OR = 2.06, 95% CI 1.3-3.1), to have fewer antenatal visits (OR = 2.17, 95% CI 1.4-3.4) and to experience discrimination in health care settings (OR = 2.69, 95% CI 2.2-3.3). CONCLUSIONS: There is a window of opportunity in antenatal care to implement targeted preventive interventions addressing potentially modifiable risk factors for poor maternal and infant outcomes. Developing the evidence base and infrastructure necessary in order for antenatal services to respond effectively to the social circumstances of women's lives is long overdue.