Objective: To explore relations between menstrual and reproductive factors and incidence of gastric cancer in a cohort study of 63,090 Norwegian women, followed over a period of 29 years.
Methods: Associations with potential risk factors were evaluated by Poisson regression analysis, considering 572 cases of gastric cancer diagnosed in women aged less than 80 years.
Results: Age at menarche showed a moderate inverse association with overall risk of gastric cancer (incidence rate ratio 0.93 per year; 95% confidence interval 0.88-1.00). No association could be established with age at menopause. Among women aged less than 50 years, an old age at first delivery was related to an increased risk, mainly of cancer of the distal part of the stomach. In multiparous women aged 50 years or more, relations with childbearing history differed significantly between subsites. Women with many pregnancies over a short period of time had an increased risk of cancer of the proximal part of the stomach. In the distal part, pregnancies over a long period seemed to confer a higher risk.
Conclusions: Relations between reproductive factors and risk of gastric cancer should be assessed separately for pre- and postmenopausal women and for subsites.