INTRODUCTION AND AIMS: This study aimed to estimate the prevalence and describe the patterns of alcohol use during pregnancy among Australian mothers. The study also aimed to examine the characteristics associated with alcohol use in pregnancy. DESIGN AND METHODS: Data comprised two representative samples of families (infant cohort = 5107 parents of 0- to 1-year-olds; child cohort = 4983 parents of 4- to 5-year-olds) from the 2005 Longitudinal Study of Australian Children. RESULTS: Alcohol use in pregnancy was reported by 37.6% of mothers of infants aged 0-1 years and 27.6% of mothers of children aged 4-5 years. The majority of women reported low level/occasional use of alcohol but, when extrapolated to population level, this equates to 131,250 children in these two age groups exposed to alcohol in utero, with over 1000 children exposed to alcohol most days and an estimated 671 infants exposed to three or more drinks per occasion. Among mothers of infants, alcohol use in pregnancy was associated with increasing maternal age, higher education, greater economic advantage and fewer physical health problems in pregnancy. Among mothers of children, maternal drinking in pregnancy was associated with increasing maternal age and smoking in pregnancy. DISCUSSION AND CONCLUSIONS: Alcohol use during pregnancy is common with around one-third of all mothers reporting use. Most women reported only occasional use, and among those who were asked, consumed one standard drink on average per occasion. Significant numbers were exposed to three or more drinks on one occasion or to alcohol most days while in utero. National guidelines recommend abstinence as no 'safe' threshold has been determined. Public health campaigns are needed to educate pregnant women regarding national guidelines.