The in utero origins of breast cancer are an increasing focus of
research. However, the long time period between exposure and disease diagnosis,
and the lack of standardized perinatal data collection makes this research
challenging. We assessed perinatal factors, as proxies for in
utero exposures, and breast cancer risk using pooled,
population-based birth and cancer registry data. Birth registries provided
information on perinatal exposures. Cases were females born in Norway, Sweden or
Denmark who were subsequently diagnosed with primary, invasive breast cancer
(n = 1419). Ten controls for each case were selected from
the birth registries matched on country and birth year (n =
14,190). Relative risks (RRs) and 95% confidence intervals (CIs) were estimated
using unconditional regression models. Breast cancer risk rose 7% (95% CI
2–13%) with every 500 g (roughly 1 s.d.) increase in birth
weight and 7% for every 1 s.d. increase in birth length (95% CI
1–14%). The association with birth length was attenuated after adjustment
for birth weight, while the increase in risk with birth weight remained with
adjustment for birth length. Ponderal index and small- and
large-for-gestational-age status were not better predictors of risk than either
weight or length alone. Risk was not associated with maternal education or age,
gestational duration, delivery type or birth order, or with several pregnancy
complications, including preeclampsia. These data confirm the positive
association between birth weight and breast cancer risk. Other pregnancy
characteristics, including complications such as preeclampsia, do not appear to
be involved in later breast carcinogenesis in young women.