Table 1

Selected baseline demographic characteristics of subjects

Risk factorsAge (yrs)20246.5 (3.3)46.5 (3.3)1.00
Pregnant ever (%)20288.187.10.76
No. of live births2023.0 (7.0)2.5 (1.9)0.35
Age at first birth (yrs) (parous only)178, 17623.7 (5.6)23.5 (7.1)0.69b
Age at menarche (yrs)20212.8 (1.4)12.9 (1.9)0.48
Weight (kg)20265.2 (11.7)65.2 (12.7)0.98
Height (cm)201, 202163.1 (6.2)162.5 (5.5)0.31
HormonesOral contraceptive use status (% yes)20261.963.40.76
Length of oral contraceptive use (months) (users only)125, 12859.6 (51.8)60.0 (46.3)0.96c
Length of oral contraceptive use (months) (all pairs)20236.9 (50.0)38.0 (46.8)0.75d
HRT use status (% yes)2027.44.50.22
Duration of HRT use (months) (users only)15, 912.0 (20.8)15.4 (29.4)0.74c
Duration of HRT use (months) (all pairs)2020.9 (6.3)0.69 (6.7)0.76d
SmokingSmoking status (% yes)20245.548.50.53
Smoking amount (cigarettes/day) (smokers only)92, 9816.4 (11.7)14.3 (11.8)0.23c
Smoking amount (cigarettes/day) (all pairs)2027.5 (11.3)7.0 (10.9)0.64d
Duration of smoking (yrs) (smokers only)92, 9817.9 (10.4)14.9 (9.4)0.04c
Duration of smoking (yrs) (all pairs)2028.2 (11.3)7.2 (9.9)0.35d
Pack-years of smoking (smokers only)92, 98358.6 (339.6)271.7 (321.9)0.07c
Pack-years of smoking (all pairs)202163.5 (290.2)131.8 (261.8)0.25d
MammographicPercent dense (%)20231.6 (20.6)36.5 (21.9)0.02
FeaturesDense area (cm2)20232.0 (22.4)37.0 (23.9)0.03
Non-Dense Area (cm2)20281.4 (48.1)78.0 (55.4)0.53
Total Area (cm2)202113.4 (46.0)115.0 (51.5)0.73
  • a Ps are based on a paired t test for continuous variables and McNemar’s test for categorical variables.

  • b P is based on a paired t test of all 202 pairs where nulliparous women are coded as 0, whereas the mean age at first birth is subtracted from the parous women. The means shown in the table have been calculated for parous women only.

  • c P is based on a two sample (unpaired) t test of applicable observations (e.g., smokers only).

  • d P is based on a paired t test of all 202 pairs where nonapplicable observations are coded as 0 (e.g., those who do not smoke are coded as 0 when comparing smoking amount).