| n | Mean % | SD | Median | IQR |
---|---|---|---|---|---|
Participant characteristics at MRI examination | |||||
 Age, months | 491 | 257.9 | 11.0 | 259.0 | 14.0 |
 BMI, kg/m2 | 487 | 23.9 | 4.4 | 23.0 | 5.1 |
 Menstrual cyclea |  |  |  |  |  |
  Luteal phase | 70 | 14.4 |  |  |  |
  Irregular periods | 50 | 10.3 |  |  |  |
  Follicular phase | 28 | 5.8 |  |  |  |
  Use of hormonal contraception | 339 | 69.6 |  |  |  |
 Left-right average breast volume, cm3 | 490 | 647.2 | 461.1 | 507.8 | 469.2 |
 Left-right average breast fat volume, cm3 | 490 | 406.3 | 349.5 | 292.2 | 327.9 |
 Left-right average breast water volume, cm3 | 490 | 240.9 | 131.2 | 209.8 | 172.4 |
 Left-right average breast percent water,b % | 491 | 41.8 | 10.3 | 41.7 | 16.0 |
Maternal characteristics at participant’s birth | |||||
 Mother’s age at menarche, years | 449 | 12.9 | 1.5 | 13.0 | 2.0 |
 Mother ever used oral contraceptive pill, % | 452 | 96.7 |  |  |  |
 Age when mother first used contraceptive pill, years | 435 | 18.8 | 3.1 | 18.0 | 3.0 |
 Mother’s height, cm | 446 | 16.6 | 6.5 | 165.1 | 7.6 |
 Mother’s age at first birth, years | 463 | 26.5 | 4.7 | 27.0 | 7.0 |
 Mother’s age at participant’s birth, years | 467 | 29.9 | 4.5 | 30.0 | 6.0 |
 Mother’s parity at participant’s birth |  |  |  |  |  |
  0 | 223 | 48.5 |  |  |  |
  1 | 161 | 35.0 |  |  |  |
  2+ | 76 | 16.5 |  |  |  |
 Mother’s pre-pregnancy BMI, kg/m2 | 430 | 22.3 | 3.0 | 21.7 | 3.5 |
 Maternal history of BC when participant was 8 years old, % | 355 | 11.6 |  |  |  |
Maternal characteristics at mammography | |||||
 Age, years | 176 | 52.7 | 3.9 | 52.0 | 5.0 |
 BMI,c kg/m2 | 165 | 24.3 | 4.7 | 23.3 | 5.4 |
 Left-right average breast area, cm2 | 176 | 295.0 | 141.3 | 266.8 | 167.1 |
 Left-right average dense area, cm2 | 176 | 63.9 | 37.3 | 59.3 | 37.6 |
 Left-right average percent density, % | 176 | 25.3 | 13.4 | 24.8 | 20.4 |
 In utero exposures |  |  |  |  |  |
  Placental weight, g | 121 | 587.1 | 132.9 | 580.0 | 160.0 |
  Absolute GWG, week 0 to delivery, kg | 422 | 12.1 | 3.9 | 12.0 | 5.0 |
  Mother drank alcohol during pregnancy, % | 459 | 74.1 |  |  |  |
  Mother smoked during pregnancy, % | 464 | 10.6 |  |  |  |
Participant characteristics at birth | |||||
 Birthweight, g | 460 | 3395.0 | 472.6 | 3400.0 | 565.0 |
 Birth length, cm | 362 | 50.5 | 2.3 | 50.8 | 2.7 |
 Head circumference, cm | 370 | 34.6 | 1.2 | 34.6 | 1.5 |
 PI,d g/cm3 | 358 | 26.4 | 4.1 | 26.1 | 3.2 |
 Gestational age,e weeks |  |  |  |  |  |
   < 39 | 93 | 19.9 |  |  |  |
  39 | 101 | 21.6 |  |  |  |
  40 | 132 | 28.3 |  |  |  |
   ≥ 41 | 141 | 30.2 |  |  |  |