The in utero origins of breast cancer are an increasing focus ofresearch. However, the long time period between exposure and disease diagnosis,and the lack of standardized perinatal data collection makes this researchchallenging. We assessed perinatal factors, as proxies for inutero exposures, and breast cancer risk using pooled,population-based birth and cancer registry data. Birth registries providedinformation on perinatal exposures. Cases were females born in Norway, Sweden orDenmark who were subsequently diagnosed with primary, invasive breast cancer(n = 1419). Ten controls for each case were selected fromthe birth registries matched on country and birth year (n =14,190). Relative risks (RRs) and 95% confidence intervals (CIs) were estimatedusing unconditional regression models. Breast cancer risk rose 7% (95% CI2–13%) with every 500 g (roughly 1 s.d.) increase in birthweight and 7% for every 1 s.d. increase in birth length (95% CI1–14%). The association with birth length was attenuated after adjustmentfor birth weight, while the increase in risk with birth weight remained withadjustment for birth length. Ponderal index and small- andlarge-for-gestational-age status were not better predictors of risk than eitherweight or length alone. Risk was not associated with maternal education or age,gestational duration, delivery type or birth order, or with several pregnancycomplications, including preeclampsia. These data confirm the positiveassociation between birth weight and breast cancer risk. Other pregnancycharacteristics, including complications such as preeclampsia, do not appear tobe involved in later breast carcinogenesis in young women.