Parental night shift work and risk of breast cancer in daughters.
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OBJECTIVES: Night shift work is common among men and women of reproductive age and maternal night shift work is linked to adverse pregnancy outcomes. It has been hypothesized that parental night shift work may influence long-term disease risk in offspring, including breast cancer. To date, no studies have directly investigated this potential association. METHODS: Our nested case-control study identified women with primary breast cancer using the Danish Cancer Registry. In total, 5,587 cases were included. For each case, 20 cancer-free female controls were selected, matched by year of birth. Inclusion criteria required that both cases and controls were born in Denmark and that information on either maternal or paternal employment history was available. Night shift work information was obtained from a job exposure matrix. RESULTS: Ever maternal night shift work prior to birth was modestly associated with an increased risk of breast cancer in daughters (OR = 1.15, 95% CI: 1.01-1.30). A modest increase in risk was also observed for night shift work in the year preceding birth (OR = 1.11, 95% CI: 0.91-1.35), and a duration-response relationship was further suggested for years of maternal night shift work prior to birth. There was no evidence of an association for paternal night shift work. Subtype analyses by estrogen receptor status resembled the overall findings. CONCLUSIONS: Maternal night shift work before childbirth was modestly associated with an increased risk of early-onset breast cancer in daughters and longer duration of night shift work preceding birth may also increase the risk. No increased risk was observed for paternal night shift work. These findings warrant confirmation in studies with more detailed exposure and lifestyle data.