Table 1.

Baseline characteristics according to parity, age at first birth, and lactationa

ParityAge at first birthbLactationb
01>2<25>25NeverEver
20,48012,43924,12325,96610,59621,34314,717
Parity ≥2 births, %69.741.057.567.6
Age at first birth ≥25 y, %43.918.622.436.2
Ever lactation, %33.845.036.252.5
Years since last birth <10, %41.449.439.060.241.552.2
Age (y), mean33.038.643.742.241.542.441.3
Age at menarche (y), mean12.312.412.412.412.412.412.3
Postmenopausal, %18.217.216.517.315.617.515.5
Family history of breast cancer, %6.76.56.46.56.36.66.2
Oral contraceptive use ≥5 y, %33.735.725.725.739.330.129.3
Ever female hormone use, %17.016.115.016.113.315.914.2
Body mass index (kg/m2), mean27.727.828.428.527.528.527.8
Alcohol ≥1 drink/d, %6.06.05.76.34.56.54.7
Smoking ≥15 cigarettes/d, %5.35.96.26.94.17.14.6
Education ≤12 y, %11.718.428.429.510.729.616.6
  • aAll means and proportions, with the exception of age, are standardized to the age distribution of the baseline cohort.

  • bRestricted to parous women.