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High estrogen exposure in utero may increase breast cancer risk later in life. However, studies of the associations between perinatal factors presumed to affect the fetal hormonal environment and breast cancer risk are inconsistent. We used data from a population-based case-control study of 2,386 incident breast cancers and 2,502 controls in Poland to evaluate risks associated with various perinatal characteristics. After adjusting for confounders, we found a significant trend (p = 0.01) of breast cancer risk with birth weight (OR = 1.54, 95% CI 1.08-2.19 for birth weights >4,000 g vs. <2,500 g). Subjects with a high birth order (> or =6) were at reduced risk (OR = 0.81, 0.61-1.06) when compared with first born subjects. Birth weight was somewhat a stronger risk predictor among subjects whose cancers were diagnosed at 50 years of age or older (OR = 1.84, 1.19-2.85) than among those with cancers diagnosed at younger ages (OR = 1.14, 0.61-2.12). Subjects whose mothers smoked during their pregnancies were at slightly higher risk than those who never smoked (OR = 1.21, 0.99-1.47), but the risk was similar to mothers who only smoked at other times (OR = 1.22, 0.81-1.84). Breast cancer risk was not related to paternal smoking, maternal age, gestational age or twin status. Our results add support to the growing evidence that some perinatal exposures may relate to breast cancer risk. Additional studies are needed to confirm associations and clarify the biologic mechanisms underlying these associations.