THE SHORT-TERM AND LONG-TERM EFFECT OF A PREGNANCY ON BREAST-CANCER RISK - A PROSPECTIVE-STUDY OF 802,457 PAROUS NORWEGIAN WOMEN

Time-related effects of a pregnancy on breast cancer risk were examine d in a population-based prospective study of 802 457 parous Norwegian women aged 20-56 years. The mean follow-up time was 16.4 years. A tota l of 4787 women were diagnosed with breast cancer. We observed a short -term increase in risk of breast cancer after a full-term pregnancy, w ith a maximum 3-4 years after delivery, followed by a long-lasting dec rease in risk. The maximum risk was about twice the risk for women who se last delivery was 20 or more years previously (incidence rate ratio = 1.99, 95% confidence interval = 1.70-2.33). Compared with nulliparo us women, those with one or two children were at higher risk in the fi rst decade after the last pregnancy, whereas those with three or more children were at lower risk in most categories of time since the last birth. The positive association between breast cancer risk and age at last birth was markedly reduced after adjustment for time since last b irth. We conclude that there is a non-linear relationship between brea st cancer incidence and time since last birth. Part of the relation wi th age at last birth may be attributed to the association with time si nce last birth.