Reproductive risk factors for breast cancer by receptor status, histology,laterality and location

It is well established that a woman's reproductive history influences her r isk of breast cancer. We examined whether the effect of reproductive histor y was similar for different sub-types of breast cancer. The study was based on a population-based cohort of 1.5 million Danish women born between 1935 and 1978, with individual information on births, Between 1978 and 1994, 10 ,790 incident cases of breast cancer were identified in a nationwide cancer registry, including detailed information on receptor status, histology, la terality and location of the tumour. Overall, the incidence of breast cance r was 13% lower in parous compared with nulliparous women. This reduction w as significantly stronger for mucinous than for ductal carcinomas and for t umours located centrally than for those non-central in the breast. Overall, the incidence in parous women increased by 10% by each 5-year postponement of their first birth. For the incidence of lobular carcinomas this increas e was significantly stronger, and for mucinous carcinomas it tended to be s tronger than for ductal carcinomas. For the incidence of centrally located tumours the increase was stronger than for non-centrally located tumours, O n average, there was a 10% decrease in breast-cancer risk by each additiona l birth. This decrease was seen in most sub-types, but not for lobular carc inomas of for centrally located tumours. According to our findings, lobular and mucinous carcinomas and centrally located tumours may have risk-factor profiles that differ from other types of breast cancer. (C) 1999 Wiley-Lis s, Inc.