Table 1 Characteristics of the study population by level of total activity

Number (%) of participants

38,913 (100)

12,180 (31.3)

15,273 (39.2)

7,591 (19.5)

3,869 (9.9)

Age at mammography screening (years), mean (SD)

55.5 (9.8)

56.6 (10.0)

56.4 (10.1)

53.6 (9.1)

51.8 (8.0)

Body mass index (kg/m²), mean (SD)

25.4 (4.2)

26.1 (4.5)

25.2 (4.0)

24.9 (4.0)

25.2 (4.2)

Age at menarche (years), mean (SD)^a

13.1 (1.5)

13.1 (1.5)

13.1 (1.5)

13.1 (1.5)

13.1 (1.5)

Absolute dense volume (cm³), mean (95% CI)

64.2 (63.9 to 64.5)

66.1 (65.6 to 66.7)

63.4 (62.9 to 63.9)

63.0 (62.3 to 63.7)

63.8 (62.8 to 64.7)

Nondense volume (cm³), mean (95% CI)

793.2 (788.6 to 797.7)

865.3 (856.8 to 873.7)

778.4 (771.4 to 785.4)

726.8 (717.1 to 736.6)

754.6 (740.0 to 769.1)

Total breast volume (cm³), mean (95% CI)

857.4 (852.7 to 862.0)

931.3 (922.7 to 940.0)

841.8 (834.6 to 849.0)

789.8 (779.9 to 799.8)

818.3 (803.3 to 833.3)

Percent dense volume (%), mean (95% CI)

9.0 (9.0 to 9.1)

8.5 (8.4 to 8.6)

9.0 (8.9 to 9.1)

9.6 (9.5 to 9.7)

9.5 (9.3 to 9.7)

Nulliparous (%)^a

12.2

15.2

11.4

9.8

11.1

Parous women only (n)

34,093

10,307

13,516

6,837

3,433

 Age at first birth (years), mean (SD)^a

27.0 (5.2)

27.0 (5.2)

27.2 (5.2)

27.2 (5.1)

26.5 (5.0)

 Number of childbirth, mean (SD)

2.2 (0.8)

2.1 (0.8)

2.2 (0.8)

2.3 (0.8)

2.3 (0.9)

Family history of breast cancer (%)^a

13.6

14.2

13.7

12.9

13.0

Education level (%)^a

 Secondary school

13.7

14.9

14.2

11.6

12.0

 High school

30.6

30.4

27.9

29.9

43.2

 University or higher

52.5

51.6

54.4

55.9

41.1

 Other

2.9

2.8

3.2

2.5

3.3

OC use, ever (%)^a

80.4

80.9

79.4

81.0

81.4

Premenopausal (%)

43.4

39.4

40.3

49.7

56.2

Postmenopausal women only (n)

22,009

7,377

9,121

3,817

1,694

 Age at menopause (years), mean (SD)

49.9 (5.3)

49.9 (5.3)

50.0 (5.4)

49.9 (5.1)

49.5 (5.4)

 HRT use (%)^a

  Never

62.2

60.8

60.7

65.3

69.0

  Past

22.4

23.3

23.9

19.9

16.5

  Current

5.1

5.2

4.8

5.7

5.6

Smoking status (%)^a

 Never

47.8

45.1

48.5

50.3

48.5

 Past

39.9

41.4

40.1

38.1

38.0

 Current

12.1

13.3

11.1

11.4

13.3

Alcohol consumption (g/day)^a

 None (%)

17.9

17.8

16.6

17.7

23.7

 0.1 to 9.9 (%)

61.2

58.2

62.4

63.9

60.7

 10.0 to 29.9 (%)

16.3

18.1

16.8

14.4

11.8

  ≥ 30.0 (%)

2.7

3.7

2.4

2.3

1.6