Parity and cumulative incidence rates of breast cancer in the Norwegian Woman and Cancer Study (NOWAC)

BackgroundThe reduced risk of breast cancer (BC) following increasing parity has been known for decades. Most prospective studies have presented the relative risk as the percentage decrease for each child during follow-up. Since the risk reduction is up to ten percent for each child, the overall lifelong BC risk reduction could be under communicated. In this study we use cumulative incidence rates (CIR) to calculate and describe the lifelong risk of BC in relation to parity. MethodsNOWAC is a prospective cohort study with 172,000 women recruited between 1991 and 2007 with follow-up through questionnaires and national registers of cancer and death. For the present analyses, we included 165 238 women with follow-up from 01.01.2000 until 31.12. 2018. We calculated CIR of BC by parity, stratified by other established BC risk factors (maternal age at first birth, breastfeeding, body mass index (BMI), smoking and alcohol consumption). ResultsAfter 17.3 years of average follow-up, 8120 women aged 35-84 years developed breast cancer. Age-specific incidence rates increased for each age group up to 60-64 years, decreased for the age group 75-79 years, and increased again among the oldest women aged 80-84. CIR for all participants up to 84 years was 11 700 per 100 000 person years (PY). In analyses stratified by parity, the CIR of BC for nullipara was 12 600 per 100 000 PY, for 1-2 children: 12 100, 3-4 children: 10 200, and 5-6 children: 8 700 per 100 000 PY. The parity-specific CIR of BC had the same pattern of decrease in analyses stratified for other BC risk factors. ConclusionCumulative incidence rates showed a consistent decrease in BC risk for each additional child. The decrease was consistent in strata of other established BC risk factors.