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Table 3 Studies assessing the association of birth order and the risk of breast cancer

From: Intrauterine environments and breast cancer risk: meta-analysis and systematic review

Type of study Ref. Year Design Cases Controls (or cohort) Country/place of study Birth order OR (95% CI) Comments
Case-control studies [51] 1967 LCC-D 229 229 USA 1 - The authors measured the mean value of birth weight instead of providing ORs (95% CIs). The mean difference between cases and matched controls was not significant (P > 0.2). They provided the frequency of each case and control in the tables and we calculated crude ORs
        2   
        3   
        4   
        5   
        6   
        ≥ 7   
  [50]a 1980 MCC 4339 12,760 USA, Japan, Slovenia, Athens, Taipei 1 Referent The risks (point estimates) only by birth order were shown in the figure in the original article.
        2 0.93  
        3 1.08  
        4 0.99  
        5 1.05  
        6 1.07  
        7 1.18  
        ≥ 8 1.02  
  [42] 1988 PCC 153 461 USA 1 Referent P for trend = 0.16
        2 0.92 (0.55–1.54)  
        3 0.98 (0.58–1.72)  
        4 0.69 (0.36–1.32)  
        ≥ 5 1.03 (0.60–1.79)  
  [46] 1991 MCC 927 2,616 USA/Wales/Japan All ages   Adjusted for age, study center, parity, age at first birth, age at menarche, height, body mass index, maternal age at birth, and menopausal status
           1 Referent  
           2 0.91 (0.73–1.02)  
           3 1.11 (0.87–1.27)  
           ≥ 4 1.09 (0.81–1.18)  
        Premenopausal   
           1 Referent  
           ≥ 2 0.76 (0.60–0.96)  
  [12] 1992 LCC 458 1,197 Sweden 1 Referent Adjusted for age and birth date
        ≥ 2 1.00 (0.76–1.32)  
  [47] 1994 PCC 2,414 9,138 USA 1 Referent Adjusted for age at first birth and number of children
        2 0.90 (0.78–1.03)  
        3 0.98 (0.84–1.14)  
        4 0.86 (0.73–1.02)  
        5 0.93 (0.78–1.11)  
        6 1.02 (0.84–1.23)  
        7 0.91 (0.73–1.14)  
        ≥ 8 0.88 (0.75–1.04)  
  [14] 1996 PCC 1,129 1,393 USA 1 Referent Adjusted for age, menopausal status, and maternal smoking; P for trend = 0.06 among both groups
        2 1.0 (0.7–1.4)  
        ≥ 3 0.8 (0.6–1.1)  
  [32] 1997 NCC 1,068 2,727 Sweden 1 Referent Adjusted for maternal age, socioeconomic status, parity, and preeclampsia or eclampsia, neonatal jaundice, severe prematurity, and twinship
        2 1.01 (0.83–1.22)  
        ≥ 3 1.01 (0.81–1.26)  
  [15] 2000 LCC 481 2,863 USA 1 1.07 (0.84–1.35) Crude ORs
        2–3 Referent  
        4–5 1.06 (0.81–1.38)  
        ≥ 6 1.50 (1.06–2.13)  
  [35] 2002 PCC 1,555 1,539 USA 1 Referent Adjusted for age and residential regions (states)
        2 1.07 (0.88–1.30)  
        3 1.07 (0.85–1.35)  
        4 1.01 (0.77–1.31)  
        5 0.66 (0.48–0.92)  
        ≥ 6 0.81 (0.62–1.08)  
  [18] 2003 LCC 881 3,423 Denmark 1 Referent Adjusted for mother's marital status, maternal age, and birth order
        ≥ 2 1.01 (0.83–1.12)  
  [36] 2004 PCC 854 785 USA All subjects   Adjusted for age, race and sampling fractions, body mass index, hosehold income, maternal age
           1 Referent  
           2–4 0.9 (0.7–1.1)  
           ≥ 5 1.0 (0.8–1.3)  
        Born ≥ 1948   
           1 Referent  
           2–4 0.9 (0.6–1.4)  
           ≥ 5 0.6 (0.3–1.3)  
  [48]a 2005 MCC 24 34 Nigeria ≤ 3 Referent Crude ORs
        ≥ 4 1.50 (0.25–8.98)  
  [20] 2005 PCC 1642 1,713 Poland 1 Referent Adjusted for age, education, age at menarche, menopausal status and age at menopause, age at first full-term pregnancy, number of full-term pregnancies, family history of breast cancer among first-degree relatives, mammography screening, and current body mass index
        2 1.07 (0.91–1.24)  
        3–5 0.99 (0.85–1.15)  
        ≥ 6 0.81 (0.61–1.06)  
         P for trend = 0.81  
  [37] 2006 PCC 1,166 2,105 USA 1 Referent Adjusted for age (years), education (years), race, body mass index, history of breast benign disease, family history of breast cancer, lactation (months), age at menarche (years), age at first full-term pregnancy (years), age at menopause (years), parity
        ≥ 2 1.27 (0.88–1.85)  
Cohort studies [31] 2006 Cohort 209 1,024 USA 1 Referent Hazard ratio for breast cancer mortality: adjusted for age at diagnosis, diagnosis year, stage at diagnosis, and birth order, with exception of birth order, which is adjusted for maternal age
        2 0.2 (0.2–0.3)  
        ≥ 3 0.2 (0.2–0.3)  
         P for trend < 0.01  
  [49]a 2001 Cohort - - Sweden Continuous scale 1.05 (1.01–1.10)  
  1. aWe did not include these studies in the meta-analysis because they employed different categories or a continuous scale, or they did not provide the numbers of cases and controls in the original article. Cohort, cohort study; LCC, case-control study with linkage with population and cancer registry data; LCC-D, case-control study with linkage with population and cancer death certification data; MCC, multicenter case-control study; NCC, nested case-control study in cohort; PCC, population-based case-control study.