Objective. To better characterize the relation between socioeconomic disadvantage and small‐for‐gestational age births (SGA). Design. Analysis of data from a mandatory population‐based surveillance system. Setting. Public or private hospitals and at home. Population. All 877,951 singleton births occurring in New South Wales, Australia, between 1994 and 2004. Methods. Multilevel models were developed to determine the factors associated with babies weighing less than the 3rd percentile for gestation and gender. Main outcome measures. Odds of SGA. Results. The risk of SGA increased with increasing socioeconomic disadvantage. Smoking accounted for approximately 40% of the increased risk associated with socioeconomic disadvantage, and delayed antenatal care approximately 5%. While the absolute rate of SGA remained stable over the study period, the odds of SGA in mothers living in the most disadvantaged areas compared to those in the least disadvantaged areas increased from approximately 1.7 to 2.2. This trend persisted after accounting for maternal smoking. The risk of SGA over this period also increased in mothers commencing antenatal care after the first trimester. After accounting for smoking, socioeconomic disadvantage and clinical conditions, mothers under 21 years of age were at reduced risk of SGA, but mothers over 35 were at increased risk. Conclusions. Socioeconomic disadvantage remains one of the dominant determinants of SGA, even in a developed country with universal insurance. This relation appears to be strengthening. Smoking patterns, inadequate antenatal care and clinical conditions partially account for this association and trend, however, most is mediated by other factors.