Baseline characteristics according to parity, age at first birth, and lactationa
| Parity | Age at first birthb | Lactationb | |||||
|---|---|---|---|---|---|---|---|
| 0 | 1 | >2 | <25 | >25 | Never | Ever | |
| 20,480 | 12,439 | 24,123 | 25,966 | 10,596 | 21,343 | 14,717 | |
| Parity ≥2 births, % | – | – | – | 69.7 | 41.0 | 57.5 | 67.6 |
| Age at first birth ≥25 y, % | – | 43.9 | 18.6 | – | – | 22.4 | 36.2 |
| Ever lactation, % | – | 33.8 | 45.0 | 36.2 | 52.5 | – | – |
| Years since last birth <10, % | – | 41.4 | 49.4 | 39.0 | 60.2 | 41.5 | 52.2 |
| Age (y), mean | 33.0 | 38.6 | 43.7 | 42.2 | 41.5 | 42.4 | 41.3 |
| Age at menarche (y), mean | 12.3 | 12.4 | 12.4 | 12.4 | 12.4 | 12.4 | 12.3 |
| Postmenopausal, % | 18.2 | 17.2 | 16.5 | 17.3 | 15.6 | 17.5 | 15.5 |
| Family history of breast cancer, % | 6.7 | 6.5 | 6.4 | 6.5 | 6.3 | 6.6 | 6.2 |
| Oral contraceptive use ≥5 y, % | 33.7 | 35.7 | 25.7 | 25.7 | 39.3 | 30.1 | 29.3 |
| Ever female hormone use, % | 17.0 | 16.1 | 15.0 | 16.1 | 13.3 | 15.9 | 14.2 |
| Body mass index (kg/m2), mean | 27.7 | 27.8 | 28.4 | 28.5 | 27.5 | 28.5 | 27.8 |
| Alcohol ≥1 drink/d, % | 6.0 | 6.0 | 5.7 | 6.3 | 4.5 | 6.5 | 4.7 |
| Smoking ≥15 cigarettes/d, % | 5.3 | 5.9 | 6.2 | 6.9 | 4.1 | 7.1 | 4.6 |
| Education ≤12 y, % | 11.7 | 18.4 | 28.4 | 29.5 | 10.7 | 29.6 | 16.6 |
aAll means and proportions, with the exception of age, are standardized to the age distribution of the baseline cohort.
bRestricted to parous women.