Preeclampsia and risk of breast cancer: A longitudinal cohort study of tumor histology.

Patients with preeclampsia have a reduced risk of breast cancer, but it is not clear if the protective effect extends to all types of breast tumors. Our objective was to determine the association of preeclampsia with ductal, lobular, and other breast cancer histology. We conducted a longitudinal cohort study of 1,459,716 patients who had pregnancies between 1989 and 2022 in Quebec, Canada. The main exposure measure was preeclampsia. The outcome was breast cancer, including ductal, lobular, and other histological subtypes diagnosed up to 34 years after childbirth. We included in situ, localized invasive, and metastatic breast cancer. We used Cox regression models to estimate hazard ratios (HR) and 95% confidence intervals (CI) for the association between preeclampsia and breast cancer histology, adjusted for maternal characteristics. Patients with preeclampsia had a lower incidence of breast cancer than patients without preeclampsia (82.1 vs. 111.7 per 100,000 person-years). Preeclampsia was associated with a 16% lower risk of breast cancer compared with no preeclampsia (HR 0.84, 95% CI 0.79-0.89), including a 14% lower risk of ductal (HR 0.86, 95% CI 0.81-0.93) and 31% lower risk of lobular tumors (HR 0.69, 95% CI 0.55-0.87). The protective association was present for in situ, localized invasive, and metastatic breast tumors. Preeclampsia was not associated with mucinous, medullary, papillary, or other breast cancer histology. We conclude that patients with preeclampsia are less likely to develop ductal and lobular breast cancer than patients with normotensive pregnancies, but do not have a reduced risk of other types of breast cancer.