It is not clear to what extent ethnic differences in the term birthweight distribution are constitutional or pathological. This study explored term birthweight heterogeneity between ethnic groups and the explanatory role of constitutional and environmental factors. As part of a prospective cohort study, the Amsterdam Born Children and their Development study, 8266 pregnant women filled out a questionnaire during early pregnancy. Ethnic groups were categorised as: native Dutch group; first and second generation Surinamese, Antillean, Turkish, Moroccan, Ghanaian and other non-Dutch groups. Only singleton livebirths with >or=37.0 weeks of gestation and with complete data were included for analysis (n = 7118). We performed linear regression analyses to estimate the association between ethnicity and, for gestational age, standardised birthweight at term, adjusted for constitutional (fetal gender, parity, maternal age, maternal height) and environmental (education, cohabitation status, maternal body mass index, smoking, alcohol consumption, depression, work stress) determinants respectively. Mean birthweight ranged from 3223 g (second generation Surinamese newborns) to 3548 g (Dutch newborns). Adjustment for constitutional factors substantially reduced the ethnic differences in birthweight, while adjustment for environmental factors provided little additional explanation. Surinamese [first generation: regression coefficient (b) = -98.3 g, P < 0.001; second generation: b = -159.3 g, P < 0.001], first generation Antillean (b = -102.0 g, P = 0.037), and Ghanaian newborns (b = -120.7 g, P = 0.001) remained significantly smaller than Dutch newborns after adjustment for all determinants. Term birthweight differences between Dutch newborns and Turkish, Moroccan and other non-Dutch newborns were largely explained by constitutional rather than environmental determinants, limiting the need for prevention. Surinamese, Antillean and Ghanaian (mainly black) newborns remained unexplainably smaller after adjustment, leaving the possibility of either unknown constitutional or pathological underlying mechanisms.