Objective To compare health behaviours during pregnancy (smoking and alcohol consumption) and after birth (initiation and duration of breast feeding) between British/Irish white mothers and mothers from ethnic minority groups; and, in mothers from ethnic minority groups, to examine whether indicators of acculturation (generational status, language spoken at home, length of residency in the United Kingdom) were associated with these health behaviours. Design Prospective nationally representative cohort study. Setting England. Participants 6478 British/Irish white mothers and 2110 mothers from ethnic minority groups. Main outcome measures Any smoking during pregnancy; any alcohol consumption during pregnancy; initiation of breast feeding; breast feeding for at least four months. Results Compared with British/Irish white mothers, mothers from ethnic minority groups were less likely to smoke (15% v 37%) or consume alcohol (14% v 37%) during pregnancy but more likely to initiate breast feeding (86% v 69%) and breast feed for at least four months (40% v 27%). Among mothers from ethnic minority groups, first and second generation mothers were more likely to smoke during pregnancy (odds ratio 3.85, 95% confidence interval 2.50 to 5.93, and 4.70, 2.49 to 8.90, respectively), less likely to initiate breast feeding (0.92, 0.88 to 0.97, and 0.86, 0.75 to 0.99), and less likely to breast feed for at least four months (0.72, 0.62 to 0.83, and 0.52, 0.30 to 0.89) than immigrants, after adjustment for sociodemographic characteristics. There were no consistent differences in alcohol consumption. Among immigrants, for every additional five years spent in the UK the likelihood of mothers smoking during pregnancy increased by 31% (4% to 66%) and they were 5% (0% to 10%) less likely to breast feed for at least four months. Conclusions After immigration, maternal health behaviours worsen with length of residency in the UK. Health professionals should not underestimate women's likelihood of engaging in risky health behaviours because of their ethnicity.