Supplemental Table 1. Age-adjusted differences in square-root percent MD by the candidate predictors and menopausal status among controls in the total dataset, NHS/NHSII

Premenopausal / Postmenopausal
Beta / SE / p-trend / Beta / SE / p-trend
Age (per year)^ / -0.06319 / 0.00943 / <.0001 / -0.05920 / 0.00642 / <.0001
Adolescent somatotype (per 1 unit) / -0.47281 / 0.03571 / <.0001 / -0.34783 / 0.03961 / <.0001
BMI at age 18 (per kg/m2)^ / -0.22392 / 0.01404 / <.0001 / -0.19904 / 0.01713 / <.0001
Current BMI (per kg/m2)^ / -0.16139 / 0.00700 / <.0001 / -0.15864 / 0.00874 / <.0001
Age at menarche (per year)^ / 0.17146 / 0.02858 / <.0001 / 0.12514 / 0.03597 / 0.0005
Nulliparous / 0.27130 / 0.12019 / 0.0241 / 0.55245 / 0.17409 / 0.0015
Parity (per child among parous) / -0.14968 / 0.04447 / 0.0008 / -0.14263 / 0.03488 / <.0001
Age at first birth (per year)^ / 0.04647 / 0.01087 / <.0001 / 0.04902 / 0.01413 / 0.0005
Height (per inch)^ / 0.03960 / 0.01646 / 0.0163 / 0.02011 / 0.02022 / 0.3202
Alcohol use (per gram/day)^ / 0.02189 / 0.00645 / 0.0007 / 0.00837 / 0.00628 / 0.1831
BBD (biopsy confirmed) / 0.48885 / 0.11062 / <.0001 / 0.53383 / 0.11514 / <.0001
BBD (unconfirmed) / 0.23420 / 0.08734 / 0.0074 / 0.41830 / 0.11120 / <.0001
Family history of breast cancer / 0.10303 / 0.14481 / 0.4769 / 0.18366 / 0.14716 / 0.2122
Duration of menopause (per year) / -- / -- / -- / 0.01769 / 0.01043 / 0.0901
Postmenopausal HT Use
Past (vs never) / -- / -- / -- / 0.38983 / 0.13328 / 0.0035
Current (vs never) / -- / -- / -- / 0.93606 / 0.11218 / <.0001


^Age centered at 53, BMI at age 18 centered at 21, current BMI centered at 25, age at menarche centered at 12, age at first birth centered at 25

Supplemental Table 2. Percent mammographic density (square-root transformed) model in the training dataset and the total dataset, NHS/NHSII in both cases and controls

Training dataset
(Cases=965/Controls=1962) / Testing dataset
(Cases=471/Controls=993) / Total dataset
(Cases=1436/Controls=2955)
Beta / SE / p-value / Beta / SE / p-value / Beta / SE / p-value
Intercept / 6.176 / 0.126 / <0.01 / 6.041 / 0.184 / <0.01 / 6.133 / 0.104 / <0.01
Case status / 0.373 / 0.056 / <0.01 / 0.461 / 0.080 / <0.01 / 0.402 / 0.046 / <0.01
Age (per year)^ / -0.045 / 0.005 / <0.01 / -0.031 / 0.007 / <0.01 / -0.041 / 0.004 / <0.01
Adolescent somatotype (per 1 unit) / -0.127 / 0.030 / <0.01 / -0.086 / 0.045 / 0.06 / -0.115 / 0.025 / <0.01
BMI at age 18 (per kg/m2)^ / -0.034 / 0.014 / 0.02 / -0.028 / 0.020 / 0.17 / -0.031 / 0.012 / 0.01
Current BMI (per kg/m2)^ / -0.125 / 0.006 / <0.01 / -0.144 / 0.009 / <0.01 / -0.131 / 0.005 / <0.01
Nulliparous / 0.210 / 0.105 / 0.05 / 0.144 / 0.148 / 0.33 / 0.183 / 0.085 / 0.03
Parity (per child among parous) / -0.086 / 0.024 / <0.01 / -0.100 / 0.035 / <0.01 / -0.090 / 0.020 / <0.01
Age at first birth (per year)^ / 0.016 / 0.010 / 0.10 / 0.020 / 0.014 / 0.15 / 0.017 / 0.008 / 0.03
Age at first birth*postmenopausal / 0.022 / 0.015 / 0.13 / 0.013 / 0.021 / 0.54 / 0.020 / 0.012 / 0.10
BBD history (biopsy confirmed) / 0.562 / 0.068 / <0.01 / 0.530 / 0.100 / <0.01 / 0.556 / 0.056 / <0.01
BBD history (unconfirmed) / 0.291 / 0.062 / 0.01 / 0.436 / 0.087 / <0.01 / 0.339 / 0.051 / 0.01
Postmenopausal status / -1.135 / 0.105 / <0.01 / -1.239 / 0.149 / <0.01 / -1.165 / 0.085 / <0.01
Past HT user (vs never) / 0.231 / 0.112 / 0.04 / 0.450 / 0.154 / <0.01 / 0.306 / 0.091 / <0.01
Current HT user (vs never) / 0.736 / 0.090 / <0.01 / 0.723 / 0.131 / <0.01 / 0.729 / 0.074 / <0.01
Root mean square error / 1.413 / 1.416 / 1.414
R-square / 0.42 / 0.43 / 0.42

NHS=Nurses’ Health Study, SE=standard error, BMI=body mass index, BBD=benign breast disease, HT use=hormone therapy use

^Age centered at 53, BMI at age 18 centered at 21, current BMI centered at 25, age at first birth centered at 25

Supplemental Figure 1. Mean measured percent mammographic density by decile of predicted percent mammographic density in the testing dataset among controls only, NHS/NHSII