• Division of Cancer Epidemiology and Genetics, National Cancer Institute, National Institutes of Health, Bethesda, MD, USA. schonfelds@fellows.iarc.fr

BACKGROUND: Nulliparity is associated with an increased risk of endometrial cancer. It is less clear whether nulliparity modifies the association between other established hormone-related risk factors. The proportion of nulliparous women has increased since the mid-1970s, but most individual studies to date have been too small to test the hypothesis that endometrial cancer risk factors may be associated more strongly with risk among nulliparous women compared with parous women.

METHODS: Data were aggregated on 26,936 postmenopausal, Caucasian, nulliparous women (360 endometrial cancers) and 146,583 postmenopausal, Caucasian, parous women (1378 endometrial cancers) from 4 US prospective studies (1979-2006). Hazard ratios (HRs) and 95% confidence intervals (CIs) were estimated in stratified analyses.

RESULTS: The risk of endometrial cancer was higher among nulliparous women than among parous women, as expected (nulliparous vs parous: HR, 1.42; 95% CI, 1.26-1.60). Stratified associations between endometrial cancer and hormone-related risk factors did not differ between nulliparous versus parous women: For both groups, oral contraceptives and earlier menopause were associated with reduced risk. The highest HRs were for obesity: A body mass index ≥30 kg/m(2) (vs <25 kg/m(2) ) increased the risk of endometrial cancer 3-fold among nulliparous women (HR, 3.04; 95% CI, 2.34-3.94) and parous women (HR, 2.88; 95% CI, 2.52-3.29).

CONCLUSIONS: The results from this large, pooled analysis of data from 4 large prospective studies suggested that nulliparity does not modify the risks of endometrial cancer associated with established hormone-related risk factors.