Skip to main content

Table 1 Characteristics of the Nurses’ Health Study (NHS) and NHSII participants at baseline (NHS: 1976; NHSII: 1989)

From: Parity, breastfeeding, and breast cancer risk by hormone receptor status and molecular phenotype: results from the Nurses’ Health Studies

  Nulliparous 1 child never breastfed 1 child breastfed 2+ children never breastfed 2+ children breastfed
Nurses’ Health Study (n = 7346) (n = 3995) (n = 3596) (n = 29,702) (n = 53,977)
 Age, years* 43.1 (7.8) 42.6 (7.8) 41.0 (8.1) 43.1 (6.9) 42.8 (7.1)
 Height, meters 1.6 (0.1) 1.6 (0.1) 1.6 (0.1) 1.6 (0.1) 1.6 (0.1)
 Body mass index at age 18, kg/m2 21.5 (3.5) 21.5 (3.5) 21.4 (3.2) 21.4 (3.0) 21.4 (2.9)
 Weight change since age 18, kg 5.7 (9.2) 6.0 (9.8) 5.9 (9.3) 6.2 (9.0) 6.3 (8.9)
 Age at menarche, years 12.5 (1.5) 12.5 (1.5) 12.5 (1.5) 12.5 (1.4) 12.6 (1.4)
 Parity, % 0 1 1 3.1 (1.3) 3.4 (1.5)
 Breastfeeding duration among parous women, %
  Never breastfed   100 0 100 0
  6 months or less   0 78 0 54
  7–12 months   0 13 0 15
  > 12 months   0 9 0 31
 Age at first birth, years   28.2 (4.9) 28.5 (5.0) 24.9 (3.0) 24.8 (3.0)
 Age at last birth, years   28.2 (4.9) 28.5 (5.0) 30.8 (4.3) 31.3 (4.3)
 Postmenopausal, % 27 23 23 19 18
 Age at menopause, years 42.9 (8.2) 44.1 (7.0) 44.5 (7.2) 46.0 (5.9) 46.5 (5.7)
 Alcohol, g/day 7.8 (12.0) 6.1 (10.4) 6.6 (11.4) 6.1 (10.3) 6.3 (10.4)
 Total physical activity, MET-hours/week 14.5 (19.9) 13.4 (19.6) 13.8 (19.0) 13.2 (19.8) 14.5 (21.8)
 History of benign breast disease, % 25 22 23 18 18
 Family history of breast cancer, % 6 6 6 6 6
Nurses’ Health Study II (n = 33,284) (n = 4790) (n = 11,471) (n = 9365) (n = 41,988)
 Age, years* 32.6 (4.8) 34.8 (4.9) 33.4 (4.6) 36.8 (4.2) 35.5 (4.0)
 Height, meters 1.7 (0.1) 1.6 (0.1) 1.6 (0.1) 1.6 (0.1) 1.6 (0.1)
 Body mass index at age 18, kg/m2 21.8 (4.1) 21.5 (3.6) 21.0 (3.0) 21.3 (3.3) 21.0 (2.8)
 Weight change since age 18, kg 6.6 (11.5) 8.6 (11.3) 7.6 (10.1) 9.2 (11.3) 8.0 (9.9)
 Age at menarche, years 12.4 (1.5) 12.3 (1.4) 12.4 (1.4) 12.3 (1.4) 12.4 (1.4)
 Parity, % 0 1 1 2.3 (0.6) 2.5 (0.7)
 Breastfeeding duration among parous women, %
  Never breastfed   100 0 100 0
  6 months or less   0 42 0 18
  7–12 months   0 37 0 18
  > 12 months   0 21 0 64
 Age at first birth, years   25.8 (4.8) 28.4 (4.3) 23.7 (3.6) 24.9 (3.5)
 Age at last birth, years   25.8 (4.8) 28.5 (4.3) 27.6 (3.8) 29.2 (3.7)
 Postmenopausal, % 3 3 2 3 2
 Age at menopause, years 37.5 (5.5) 38.0 (3.6) 38.7 (3.7) 37.7 (3.1) 37.8 (3.7)
 Alcohol, g/day 4.0 (7.0) 2.8 (5.7) 2.9 (5.3) 2.4 (5.2) 2.3 (4.7)
 Total physical activity, MET-hours/week 29.7 (42.1) 23.3 (34.2) 23.4 (34.8) 22.0 (33.5) 21.6 (30.8)
 History of benign breast disease, % 9 9 8 8 7
 Family history of breast cancer, % 6 6 6 5 6
  1. Values are means (SD) or percentages and are standardized to the age distribution of the study population. Values of polytomous variables may not sum to 100% due to rounding
  2. *Value is not age adjusted
  3. Age at natural menopause or bilateral oophorectomy