Table 1

Characteristics of the study population in the Women’s Lifestyle and Health Study

CharacteristicsStudy populationBreast cancer cases
No. (%)No. (%)
Country of residence
 Norway55,710 (54.1)541 (53.7)
 Sweden47,317 (45.9)467 (46.3)
Age at enrollment (yrs)
 30–3412,332 (12.0)49 (4.9)
 35–3931,862 (30.9)218 (21.6)
 40–4429,902 (29.0)310 (30.8)
 45–4928,931 (28.1)431 (42.8)
 Mean age (±SD)40.7 (5.1)42.7 (4.7)
Mean height (±SD)166.3 (5.7)166.9 (5.6)
Mean weight (±SD)64.3 (10.6)63.8 (10.7)
BMI (kg/m2)
 Less than 18.52,159 (2.1)16 (1.6)
 18.5–2472,542 (72.5)753 (76.4)
 25–2920,264 (20.3)178 (18.1)
 30 or more5,111 (5.1)38 (3.9)
 Mean BMI (±SD)23.2 (3.6)22.9 (3.5)
Age at menarche (yrs)
 12 or less33,181 (32.7)306 (30.9)
 1329,946 (29.5)302 (30.5)
 14 or more38,329 (37.8)382 (38.6)
 Mean age at menarche (±SD)13.1 (1.4)13.1 (1.3)
Postmenopausal at entry4,166 (4.0)41 (4.1)
Ever used hormonal replacement therapy3,389 (3.9)58 (5.6)
History of breast cancer in mother/sister(s)4,997 (4.9)104 (10.3)
Parity
 Nulliparous12,169 (11.8)136 (13.5)
 One child14,536 (14.1)155 (15.4)
 Two children44,907 (43.6)449 (44.5)
 Three or more children31,415 (30.5)268 (26.6)
Mean number of children (±SD)b2.0 (1.1)1.9 (1.1)
Age at first birth (yrs)
 Less than 21 y20,928 (23.0)162 (18.6)
 21–2432,756 (36.0)308 (35.3)
 25 or more37,309 (41.0)402 (46.1)
Mean age at first birth (±SD)24.0 (4.4)24.7 (4.6)
Mean duration of breast-feeding (±SD)11.8 (10.8)11.4 (10.3)
  • a BMI, weight (kg)/height (m2).

  • b Among all women.