DUAL EFFECT OF PARITY ON BREAST-CANCER RISK

This study examined whether breast cancer risk increased for a short p eriod after childbirth, but decreased after a longer period of time. D ata from an international case-control study on breast cancer conducte d in the 1960s were used to study the modifying effect of age at enrol ment on the relationship between parity and breast cancer risk, compar ing first uniparous with nulliparous women, and then biparous versus u niparous women. The statistical analysis was performed by modelling th rough multiple logistic regression, adjusting for study site, age at m enarche, menopausal status and obesity index. Comparing uniparous with nulliparous women, an early age at birth seems to be protective for a ll periods after birth, whereas a late age at birth imparts a higher r isk than nulliparity in the period immediately after birth, which decl ines with the passage of time. The modification effect by age was not apparent when biparous women with different age at second birth were c ompared with uniparous women. The results support the hypothesis that pregnancy oestrogens impart a transient increase of maternal breast ca ncer risk when the full-term pregnancy occurs late in a woman's life.