BACKGROUND:Understanding the prevalence of, and factors associated with, caesarean birth among immigrant populations is essential for appropriate antenatal and postnatal resource allocation. AIMS:To compare rates of caesarean birth between one of the largest immigrant populations of women giving birth in Australia (Vietnamese-born women) with those of Australian-born women and investigate the odds of unplanned caesarean in these women, controlling for maternal characteristics, pregnancy complications and labour management factors. METHODS:Cross-sectional analysis of data on singleton births in Victoria, Australia; using data from the routinely collected Victorian Perinatal Data Collection. Descriptive analyses comparing maternal and obstetric factors for Vietnamese-born women with Australian-born women were undertaken. Using the more recent nine years of routinely collected data, multivariable logistic regression explored the association between unplanned caesarean birth and maternal country of birth, adjusted for maternal and obstetric factors, admission status and time (n=468,131). This association was also explored for 'standard primiparae' (n=69,039). FINDINGS:Planned and unplanned caesarean births increased dramatically in both Australian-born and Vietnamese-born women between 1984 and 2007. After adjustment for obstetric and maternal factors, Vietnamese-born women were at greater odds of an unplanned caesarean birth compared to Australian-born women (adjusted odds ratio=1.32, 95% confidence interval=1.25-1.40). These greater odds were also found among the 'standard primiparae' (adjusted odds ratio=1.22, 95% confidence interval=1.07-1.40). CONCLUSION:Factors other than clinical risk appear to predispose Vietnamese-born women to unplanned caesarean birth. These may include intrapartum communication, length of residence and familiarity with care systems, and variations in care practices for Vietnamese women.