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Table 2 Description of the Danish Nurse Cohort (n = 21,831) at the time of recruitment in 1993 or 1993 and by birth cohort

From: Active smoking and risk of breast cancer in a Danish nurse cohort study

Birth Cohort 1900–1934 N = 5179 1935–1944 N = 6707 1945–1949 N = 4564 1950–1955 N = 5381
Age
 Mean (SD) age at baseline (years) 65.3 (5.8) 53.1 (3.0) 47.7 (1.9) 46.4 (1.5)
Reproductive Factors
 Mean (SD) age at menarche 14.0 (1.5) 13.6 (1.6) 13.4 (1.5) 13.2 (1.5)
 Nulliparous, n (%) 1401 (27.1) 764 (11.4) 431 (9.4) 574 (10.7)
 Mean (SD) number of children in parous women 2.5 (1.0) 2.4 (0.9) 2.3 (0.8) 2.3 (0.8)
 Mean (SD) age at first birth in parous women 27.9 (4.1) 25.7 (3.6) 25.0 (3.6) 25.6 (4.2)
 Postmenopausal, n (%) 5170 (99.8) 5060 (75.4) 1240 (27.2) 906 (16.8)
Body Mass Index (BMI)
 Mean (SD) BMI (kg/m2) 23.8 (3.5) 23.7 (3.4) 23.5 (3.4) 23.8 (3.6)
 BMI < 18.5 kg/m2, n (%) 199 (3.8) 148 (2.2) 100 (2.2) 82 (1.5)
 BMI 18.5–24.9 kg/m2, n (%) 3368 (65.0) 4657 (69.4) 3293 (72.2) 3752 (69.7)
 BMI 25–29.9 kg/m2, n (%) 1315 (25.4) 1527 (22.8) 944 (20.7) 1193 (22.2)
 BMI ≥ 30 kg/m2, n (%) 297 (5.7) 375 (5.6) 227 (5.0) 354 (6.6)
Alcohol consumption
 Does not drink alcohol, n (%) 1278 (24.7) 980 (14.6) 516 (11.3) 546 (10.1)
 Moderate drinker (1–14 drinks/week), n (%) 2900 (56.0) 4177 (62.3) 2923 (64.0) 3533 (65.7)
 Heavy drinker (> 14 drinks/week), n (%) 1001 (19.3) 1550 (23.1) 1125 (24.6) 1302 (24.2)
Leisure time physical activity
 Low physical activity, n (%) 467 (9.0) 392 (5.8) 250 (5.5) 319 (5.9)
 Medium physical activity, n (%) 1090 (21.0) 1809 (27.0) 1406 (30.8) 1600 (29.7)
 High physical activity, n (%) 3622 (69.9) 4506 (67.2) 2908 (63.7) 3462 (64.3)
Shift Work
 Working night shifts, n (%) 191 (12.5) 453 (7.4) 178 (4.0) 145 (2.7)
Hormone Use
 Ever used oral contraceptives, n (%) 2082 (40.2) 2490 (37.1) 779 (17.1) 600 (11.2)
 Ever used hormone therapy, n (%) 1143 (22.1) 3544 (52.8) 3462 (75.9) 4552 (84.6)
Smoking status
 Never smoker, n (%) 1481 (28.6) 2487 (37.1) 1840 (40.3) 2115 (39.3)
 Former smoker, n (%) 1812 (35.0) 1716 (25.6) 1242 (27.2) 1787 (33.2)
 Current smoker, n (%) 1886 (36.4) 2504 (37.3) 1482 (32.5) 1479 (27.5)
 Ever smoker, n (%) 3698 (71.4) 4220 (62.9) 2724 (59.7) 3266 (60.7)
Smoking Intensity
 Mean (SD) smoking durationa (years) 29.4 (14.5) 23.4 (11.3) 19.8 (9.5) 19.4 (9.2)
 Mean (SD) smoking intensitya (g/day) 12.1 (9.0) 13.0 (8.8) 12.5 (7.5) 12.0 (7.5)
 Mean (SD) pack-yearsa 19.5 (18.6) 16.9 (14.9) 13.5 (11.1) 12.8 (11.0)
Initiation of smoking
 Mean age (SD) at smoking initiationa (years) 22.6 (6.9) 21.1 (5.3) 19.6 (3.9) 17.8 (3.3)
Initiation of smoking in 18,661 parous women
 Parous never smoker, n (%) 1078 (28.5) 2246 (37.8) 1684 (40.8) 1930 (40.2)
 Parous ever smoker, n (%) 2700 (71.5) 3697 (62.2) 2449 (59.3) 2877 (59.9)
 Parous/around 1st birth or later, n (%) 430 (8.3) 632 (9.4) 259 (5.7) 136 (2.5)
 Parous/before 1st childbirth, ≤ 10 pack-years, n (%) 2047 (39.5) 2824 (42.1) 2037 (44.6) 2459 (45.7)
 Parous/before 1st childbirth, > 10 pack-years, n (%) 223 (4.3) 241 (3.6) 153 (3.4) 282 (5.2)
Initiation of smoking in 3170 nulliparous women
 Nulliparous never smoker, n (%) 403 (28.8) 241 (31.5) 156 (36.2) 185 (32.2)
 Nulliparous ever smoker, n (%) 998 (71.2) 523 (68.5) 275 (63.8) 389 (67.8)
 Nulliparous, before age 21b years, n (%) 442 (8.5) 295 (4.4) 187 (4.1) 340 (6.3)
 Nulliparous, age 21 years or later, n (%) 556 (10.7) 228 (3.4) 88 (1.9) 49 (0.9)
Initiation of smoking in 12,376 postmenopausal women
 Postmenopausal never smoker, n (%) 1477 (28.6) 1757 (34.7) 454 (36.6) 300 (33.1)
 Postmenopausal ever smoker, n (%) 3693 (71.4) 3303 (65.3) 786 (63.4) 606 (66.9)
 Before menopause, n (%) 3633 (70.3) 3281 (64.8) 782 (63.1) 605 (66.8)
 After menopause, n (%) 60 (1.2) 22 (0.4) 4 (0.3) 1 (0.1)
  1. SD standard deviation; ain ever smokers. b21 is mean age of smoking initiation in the cohort