Table 1.
Characteristics of the Nurses’ Health Study (NHS) and NHSII participants at baseline (NHS: 1976; NHSII: 1989)
| 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 |
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
*Value is not age adjusted
†Age at natural menopause or bilateral oophorectomy