Transient increase in breast cancer risk after giving birth: postpartum period with the highest risk (Sweden).

OBJECTIVE: Identify time-points when the elevated postpartum maternal breast cancer risk peaks. METHODS: A case-control study nested within the Swedish Fertility Register included 34,018 breast cancer cases from the Swedish Cancer Register between 1961 and 1995. From the Fertility Register, 170,001 eligible subjects matched to the cases by age were selected as controls. Analysis contrasted risk between uniparous (7084 cases and 31,703 controls) and nulliparous (5411 cases and 22,580 controls) women and between biparous (13,239 cases and 65,858 controls) and uniparous women. Logistic regression analysis included indicator variables representing each year of age, ages at delivery, and time since delivery. RESULTS: Comparing uniparous with nulliparous women the transient increase in maternal breast cancer risk peaked 5 years following delivery (odds ratio= 1.49, 95% confidence interval 1.01-2.20) and leveled off 15 years postpartum. Biparous women had a transient increase in risk that was lower at its peak than that of uniparous women, occurring about 3 years following second delivery. CONCLUSIONS: A time window of 5 years postpartum when maternal breast cancer risk is highest was observed. Establishing timing of peak transient increase in postpartum breast cancer risk may define the latent period required for pregnancy hormones in promoting progression of breast cells that have undergone early stages of malignant transformation.