Menstrual and reproductive factors may increase breast cancer risk through a pathway that includes increased mammographic density. We assessed whether known or suspected menstrual and reproductive breast cancer risk factors were cross-sectionally associated with mammographic density, by measuring area of radiographic density and total breast area on mammograms from 801 participants in the Study of Women's Health Across the Nation (SWAN), a multi-ethnic cohort of pre- and early perimenopausal women. From multivariable linear regression, the following menstrual or reproductive factors were independently associated with percent mammographic density (area of dense breast/breast area): older age at menarche (beta=10.3, P<0.01, for >13 vs. <12 years), premenstrual cravings and bloating (beta=-3.36, P=0.02), younger age at first full-term birth (beta=-8.12, P<0.01 for <or=23 years versus no births), greater number of births (beta=-6.80, P<0.01 for >or=3 births versus no births), and premenopausal status (beta=3.78, P<0.01 versus early perimenopausal). Only number of births remained associated with percent density after adjustment for age, race/ethnicity, study site, body mass index (BMI), and smoking. In addition, stratified analyses revealed that the association with number of births was confined to women within the lowest BMI tertile (beta=-12.2, P<0.01 for >or=3 births versus no births). Our data support a mechanism for parity and breast cancer that involves mammographic density among pre- and early perimenopausal women that may be modified by body size.