Troisi R, et al. - Given that preeclampsia is related to a lower risk of maternal breast cancer, researchers performed this population-based case-control study to assess causality, biological mechanisms, and the association of other hypertensive conditions to risk. Participants were parous women with breast cancer (n=116,196) and 10 matched controls per case (n=1,147,192) sampled from the birth registries. Findings showed a link between hypertension diagnosed before pregnancy, gestational hypertension, and preeclampsia, with attenuated breast cancer risk. Comparable results were seen after adjusting for smoking and maternal body mass index prior to first pregnancy, and were generally comparable stratified by parity, age at breast cancer diagnosis, time since first and last birth, sex of the offspring and calendar time. No other pregnancy complication appeared to be related to breast cancer risk, except for retained placenta.