Table 1.
Age-adjusted proportion of person-years for breast cancer risk factors according to oral contraceptive use, among women followed-up in the Nurses’ Health Study II population (n= 116,413 women aged 24–43 years in 1989)
| Oral Contraceptive Use | |||
|---|---|---|---|
| Never | Past | Current | |
| (%) | (%) | (%) | |
| Age at Menarche (yrs): | |||
| <12 | 24 | 25 | 22 |
| 13 | 27 | 27 | 28 |
| ≥14 | 19 | 17 | 19 |
| Parity: | |||
| None | 32 | 19 | 38 |
| 1 | 12 | 17 | 19 |
| 2 | 29 | 39 | 31 |
| 3 | 18 | 19 | 9 |
| ≥4 | 9 | 6 | 2 |
| Age at first birth (yrs): | |||
| ≤24 | 39 | 42 | 38 |
| >30 | 11 | 9 | 12 |
| Family history of breast cancer: | |||
| Yes | 7 | 6 | 5 |
| Benign breast disease: | |||
| Yes, biopsy confirmed | 12 | 13 | 8 |
| BMI (kg/m2): | |||
| <21 | 21 | 19 | 27 |
| 21-<23 | 19 | 20 | 24 |
| 23-<25 | 16 | 17 | 18 |
| 25-<30 | 21 | 22 | 19 |
| ≥30 | 19 | 17 | 11 |
| Alcohol intake: | |||
| None | 54 | 44 | 38 |
| 1-<5 g/day | 22 | 24 | 29 |
| 5-<15 | 11 | 14 | 19 |
| ≥15 g/day | 2 | 4 | 4 |
| Animal fat intake (quintiles): | |||
| Quintile 1 (lowest) | 17 | 15 | 15 |
| Quintile 3 | 15 | 15 | 14 |
| Quintile 5 (highest) | 14 | 15 | 13 |
| Smoking: | |||
| Never | 77 | 62 | 71 |
| Past | 16 | 25 | 20 |
| Current | 7 | 13 | 9 |
| Ovulatory infertility: | |||
| Yes | 7 | 7 | 3 |
| Menstrual cycle irregularity | |||
| Regular | 77 | 73 | 72 |
| Very irregular | 7 | 7 | 7 |
| Total breast feeding duration (months): | |||
| none | 13 | 16 | 15 |
| <1 | 4 | 5 | 5 |
| 4–6 | 9 | 11 | 13 |
| ≥12 | 52 | 41 | 36 |