BACKGROUND: The mean birth weight of offspring of Bangladeshi, Indian and Pakistani women tends to be among the lowest of any ethnic groups regardless of country of residence. However, it is unclear whether the mean birth weight of South Asian offspring born in England and Wales is higher among those whose mothers were themselves born in England and Wales compared to those whose mothers were born in the Indian sub-continent. METHODS: We used cross-sectional data from a unique linkage of routine records for the whole of England and Wales (2005-2006, n=861 654) to estimate mean birth weights of the live singleton offspring of Bangladeshi, Indian, Pakistani or White British ethnicity according to whether maternal place of birth was England and Wales or the Indian sub-continent. RESULTS: Offspring of women born in the Indian sub-continent were slightly heavier at birth than offspring of South Asian women born in England and Wales even after adjustment for gestational age, maternal age and parity (Bangladeshi 28 g, 95% CI 10 to 46; Indian 31 g, 95% CI 20 to 42; Pakistani 21 g, 95% CI 12 to 29). CONCLUSIONS: There is no indication that the mean birth weight of South Asian offspring of women born in England and Wales is higher than the mean birth weight of those whose mothers were born in the Indian sub-continent. This suggests a shared physiological tendency for down-regulation of fetal growth transmissible across generations. Within the UK, there is unlikely to be any appreciable increase in mean birth weight of South Asian babies over the next few decades.