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Table 2 Prevalence of Reproductive Risk Factors of Female Kaiser Permanente Members Aged 40–65 In Marin County and East/Central Costa County, California

From: Differences in reproductive risk factors for breast cancer in middle-aged women in Marin County, California and a sociodemographically similar area of Northern California

  Marin East/Central Contra Costa p-value
  Base No. Percent (95% CI) Base No. Percent (95% CI)  
Menstrual history      
   Age at onset of menarche 415     p = .91
   < 12 yrs   17.0 (13.4 – 20.7)   17.1 (13.1 – 21.1)  
   12–13 yrs   61.5 (56.7 – 66.2)   62.7 (57.5 – 67.8)  
   ≥ 14 yrs   21.5 (17.5 – 25.5)   20.2 (16.0 – 24.4)  
Current Menstrual Status 414   347   p = .16
   Pre-menopausal   28.1 (23.6 – 47.1)   34.3 (29.2 – 39.5) p = .07
   Peri-menopausal   14.2 (10.8 – 17.5)   14.5 (10.7 – 18.3) p = .89
   Post-menopausal (any reason)   57.7 (52.9 – 62.5)   51.1 (45.8 – 56.5) p = .08
Natural menopause   47.8 (42.9 – 52.6)   36.2 (31.1 – 41.3) p = .002
Surgical menopause   9.6 (6.9 – 12.8)   14.9 (11.2 – 18.7) p = .03
Age (mean) at menopause 217 48.4 yr (se = 0.4) 175 47.3 (se = 0.5) p = .09
Contraceptive and HRT Use      
   Ever used oral contraceptives 426 84.8 (81.4 – 88.2) 361 81.6 (77.5 – 85.6) p = .23
   Ever used HRT for menopause 423 39.3 (35.1 – 44.6) 358 43.6 (38.3 – 58.9) p = .32
Pregnancy History 422   361   p = .12
   Never pregnant   14.8 (11.4 – 18.2)   12.2 (8.8 – 15.7)  
   One pregnancy   16.7 (12.9 – 20.2)   12.8 (9.2 – 16.1)  
   ≥ 2 pregnancies   68.5 (64.0 – 73.0)   75.0 (70.4 – 79.5)  
Parity      
   Nulliparous1 419 29.2 (24.8 – 33.6) 359 15.4 (11.6 – 19.2) p < .0001
   Uniparous   19.3 (15.4 – 23.1)   19.1 (14.9 – 23.2)  
   Multiparous   51.5 (46.7 – 56.3)   65.5 (60.6 – 70.6) p < .0001
Age at First Live Birth 420   359   p < .0001
   <20 yrs   7.1 (4.6 – 9.6)   9.2 (6.3 – 12.2)  
   20–24 yrs   17.9 (14.2 – 21.5)   24.9 (20.5 – 29.6)  
   25–29 yrs   22.3 (18.3 – 26.3)   24.9 (20.4 – 29.5)  
   No live birth before age 30 2   52.7 (47.9 – 57.5)   40.8 (35.6 – 46.0) p = .0011
Total months of breastfeeding 419   360   p = .45
   0 (includes no live births)   43.0 (38.2 – 47.7)   37.3 (32.2 – 42.4)  
   1–12   30.2 (25.7 – 34.6)   34.1 (29.1 – 39.1)  
   13–24   16.3 (12.7 – 19.8)   17.6 (13.6 – 21.7)  
   >24   10.5 (7.6 – 13.5)   10.9 (7.6 – 14.3)  
High risk Gail Score (≥ 1.7) 3 403 24.9 (20.7 – 29.1) 347 20.9 (16.7 – 25.2) p = .17
  1. 1 Includes women who were never pregnant or never had a live birth
  2. 2 Includes women who never had a live birth
  3. 3 Calculated only for women with no history of breast cancer