Table 1.
Characteristics of participant sisters in the Two Sister Study
| Characteristic | Control sisters (n = 1669) | Case sisters (n = 1422) |
|---|---|---|
| Race, No. (%) | ||
| Non-Hispanic white | 1487 (89.1) | 1253 (88.1) |
| Others | 182 (10.9) | 169 (11.9) |
| Education, No. (%) | ||
| High school or less | 217 (13.0) | 177 (12.4) |
| Some college | 533 (31.9) | 410 (28.8) |
| Bachelor degree | 525 (31.5) | 480 (33.8) |
| Master or doctoral degree | 394 (23.6) | 355 (25.0) |
| Age at menarche‡, mean (SD), y | 12.8 (1.5) | 12.6 (1.5) |
| Nulliparous, No. (%) | 358 (21.5) | 304 (21.4) |
| Age at first birth among parous women, mean (SD), y | 26.8 (5.4) | 27.7 (5.2) |
| Total duration of breastfeeding, mean (SD), weeks | 46.3 (70.8) | 44.5 (64.4) |
| Cigarette smoking‡, No. (%) | ||
| Never-smoker | 1044 (62.6) | 859 (60.4) |
| <1 pack-year | 148 (8.9) | 157 (11.0) |
| 1–<10 pack-years | 235 (14.1) | 217 (15.3) |
| ≥10 pack-years | 240 (14.4) | 189 (13.3) |
| Alcohol drinking in the 10 years preceding index age‡, No. (%) | ||
| Non-drinker | 317 (19.3) | 263 (18.6) |
| 0.1-<5 g/d | 821 (50.1) | 725 (51.2) |
| 5-<10 g/d | 241 (14.7) | 205 (14.5) |
| ≥10 g/d | 261 (15.9) | 222 (15.7) |
| Body mass index at ages 30–39 y‡ | ||
| Underweight and normal (<25.0 kg/m2) | 1210 (72.7) | 1035 (73.1) |
| Overweight (25.0–29.9 kg/m2) | 312 (18.7) | 283 (20.0) |
| Obesity (≥30.0 kg/m2) | 143 (8.6) | 98 (6.9) |
| Use of hormonal birth control‡, No. (%) | ||
| Never user | 163 (9.8) | 135 (9.5) |
| <10 y | 889 (53.4) | 700 (49.3) |
| ≥10 y | 590 (35.4) | 568 (40.0) |
| Unknown duration | 24 (1.4) | 17 (1.2) |
| Hormone replacement therapy‡, No. (%)† | ||
| Never user | 1499 (89.9) | 1346 (94.8) |
| Estrogen only | 113 (6.8) | 50 (3.5) |
| Combined estrogen and progesterone | 55 (3.3) | 24 (1.7) |
| Menopausal status‡, No. (%) | ||
| Premenopausal | 1396 (83.7) | 1269 (89.2) |
| Postmenopausal | 147 (8.8) | 76 (5.3) |
| Premenopausal hysterectomy, with retained ovarian tissue | 124 (7.4) | 77 (5.4) |
| Interval between interview and index age, y | ||
| <1 | 587 (35.2) | 0 (0) |
| l-<5 | 491 (29.4) | 1110 (78.1) |
| 5-<10 | 423 (25.3) | 270 (19.0) |
| ≥10 | 168 (10.1) | 42 (3.0) |
All time-varying variables (parity, age at first birth among parous women, total duration of breastfeeding, use of hormonal birth control, cigarette smoking, alcohol drinking and menopausal status) were defined in reference to the index age; index age was defined as the minimum of age at diagnosis for the case sister and the age(s) at interview of her control sister(s).
‡
Missing data: household income per person (n=81), age at menarche (n=1), parity (n=1), duration of breastfeeding (n=3), body mass index at age 30 (n=10), hormonal birth control (n=5), smoking (n=2), alcohol drinking (n=36), menopausal status (n=2).