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Table 2 Association between parity (ever versus never) and BC subtypes for age overall and for specific ages (35, 55 and 75 years)

From: Reproductive profiles and risk of breast cancer subtypes: a multi-center case-only study

  Age at BC diagnosis Odds ratio (95% CI) P value P value interaction parity × age
Luminal A-like All ages 1.00 (Ref.)   
Luminal B-like All ages 0.90 (0.77–1.05) 0.18  
Luminal HER2-like All ages 1.04 (0.88–1.24) 0.62  
HER2-like All ages 1.04 (0.83–1.29) 0.73  
TNBC All ages 1.38 (1.16–1.65) 0.0004  
Luminal A-like At 35 years 1.00 (Ref.)   
At 55 years 1.00 (Ref.)   
At 75 years 1.00 (Ref.)   
Luminal B-like At 35 years 0.95 (0.64–1.43) 0.82  
At 55 years 0.91 (0.65–1.26) 0.55  
At 75 years 0.89 (0.64–1.24) 0.48 0.99
Luminal HER2-like At 35 years 1.48 (1.01–2.16) 0.046  
At 55 years 1.35 (0.92–1.99) 0.13  
At 75 years 0.72 (0.45–1.14) 0.16 0.037
HER2-like At 35 years 1.38 (0.86–2.20) 0.18  
At 55 years 1.56 (0.98–2.47) 0.06  
At 75 years 0.59 (0.33–1.05) 0.07 0.030
TNBC At 35 years 1.40 (0.98–2.00) 0.06  
At 55 years 1.80 (1.24–2.63) 0.002  
At 75 years 1.67 1.01–2.76) 0.046 0.076
  1. A baseline-category logits model was fitted with breast cancer (BC) subtype as e esponse variable taking luminal A BC as a reference category, and parity and age at diagnosis (as a continuous variable) as explanatory variables. Age was modeled non-linearly using cubic splines (five knots). The p value was 0.0149 for interaction effect between parity and age. A random intercept was introduced to account for clustering by study. Interactions between parity and age at BC diagnosis in the probability of developing a specific BC subtype were tested by logistic regression models with binary outcome (1 for the BC subtype and 0 for luminal A-like BC as the reference subtype). The interaction between age and parity in BC subtype risk was tested across all ages and, because age was modeled non-linearly, also specifically at age 35, 55 and 75 years. BC breast cancer, HER2 human epidermal growth factor receptor-2, TNBC triple negative breast cancer