Incidence of cancer among grand multiparous women in Finland with special focus on non-gynaecological cancers: A population-based cohort study

Background. Many studies have previously revealed evidence of an association between grand multiparity (five or more deliveries) and gynaecological cancer. Oestrogen has an impact on cancer formation and the amount of circulating oestrogen is significantly higher during pregnancy. Also the lifestyle of grand multiparous women differs somewhat from the average population. Considering these factors it is plausible that also non-gynaecological cancers are associated with multiparity. The aim of our study was to determine cancer incidence among grand multiparous women, with special attention to non-gynaecological cancers. Material and methods. All 102 541 women alive in 1974-2011 and having had at least five deliveries were identified in the Finnish Population Register and followed up for cancer incidence through the Finnish Cancer Registry to the end of 2011. Standardised incidence ratios (SIRs) were defined as ratios between observed and expected numbers of cases, the latter ones based on incidence in the entire Finnish female population. Results. The overall incidence of non-gynaecological cancers was the same as in the reference population (SIR 0.98, 95% confidence interval 0.90-1.06). The incidence of cancers of the gall-bladder (SIR 1.42, 1.26-1.58), biliary tract (1.19, 1.04-1.35) and kidney (1.22, 1.14-1.31) was increased. There were significantly fewer cases than expected of urinary bladder cancer (SIR 0.70, 0.61-0.78), lung cancer (0.87, 0.81-0.92), colon cancer (0.94, 0.89-0.99) and all types of skin cancers. As a consequence of the decreased incidence of gynaecological cancers (SIR 0.74, 0.71-0.77) and breast cancer (0.60, 0.58-0.61), the SIR for cancer overall was 0.84 (0.83-0.85). Conclusion. The study demonstrated that grand multiparous women have a similar overall risk of non-gynaecological cancers as other women, despite significant differences in some specific forms of cancer.