Dual effect of parity on breast cancer risk.

This study examined whether breast cancer risk increased for a short period after childbirth, but decreased after a longer period of time. Data from an international case-control study on breast cancer conducted in the 1960s were used to study the modifying effect of age at enrolment on the relationship between parity and breast cancer risk, comparing first uniparous with nulliparous women, and then biparous versus uniparous women. The statistical analysis was performed by modelling through multiple logistic regression, adjusting for study site, age at menarche, menopausal status and obesity index. Comparing uniparous with nulliparous women, an early age at birth seems to be protective for all periods after birth, whereas a late age at birth imparts a higher risk than nulliparity in the period immediately after birth, which declines with the passage of time. The modification effect by age was not apparent when biparous women with different age at second birth were compared with uniparous women. The results support the hypothesis that pregnancy oestrogens impart a transient increase of maternal breast cancer risk when the full-term pregnancy occurs late in a woman's life.