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Table 6 Studies assessing the association of twinship with risk for 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 Category OR (95% CI) Comments
Case-control studies [63]a 1992 MCC 870 2,741 UK, USA Singleton Referent Adjusted for age, study center, parity, age at first birth, age at menarche, height, body mass index, maternal age at birth, birth order, and menopausal status
        Twinship 1.40 (0.77–2.55)  
        Singleton Referent  
        Monozygote twin 1.30 (0.58–2.92)  
        Dizygote twin 1.54 (0.64–3.71)  
  [14] 1996 PCC 1,134 1,380 USA Age 21–45   Adjusted for age, menopausal status, and maternal smoking
           Singleton Referent  
           Twinship 0.6 (0.3–1.3)  
        Age 50–64   
           Singleton Referent  
           Twinship 0.9 (0.4–2.2)  
  [58]a 1997 PCC 2,150 1,961 USA Singleton Referent Adjusted for age, study site, family history of breast cancer, breast biopsy, a combination variable including number of full-term births and age at first full-term pregnancy, age at menarche, menopausal status, body mass index, average lifetime alcohol consumption, and the number of mammograms
        Twinship 1.6 (1.0–2.7)  
        Singleton Referent  
        Monozygote twin 1.39 (0.7–2.6)  
        Dizygote twin 2.06 (1.0–4.5)  
  [32] 1997 NCC 1,068 2,727 Sweden Singleton Referent Adjusted for maternal age, matermal socioeconomic status, maternal parity, maternal pre-eclampsia or eclampsia, neonatal jaundice, severe prematurity, twin, and birth weight
        Twinship 1.3 (0.8–2.1)  
        Singleton Referent  
        Monozygote twin 0.7 (0.2–2.2)  
        Dizygote twin 1.5 (0.8–2.7)  
  [15] 2000 LCC 481 2,863 USA Singleton Referent Crude ORs
        Twinship 1.04 (0.51–2.11)  
  [43] 2001 LCC 319 768 USA Singleton Referent Crude ORs
        Twinship 1.6 (0.2–10.1)  
  [45] 2004 LCC 2,522 10,052 USA Singleton Referent Adjusted for age and maternal age at first birth
        Twinship 1.77 (1.05–2.97)  
  [20] 2005 PCC 2,338 2,476 Poland Singleton Referent Adjusted for age, education, age at menarche, menopausal status and age at menopause, age at first full-term pregnancy, number of full-term pregnancy, family history of breast cancer among first-degree relatives, mammography screening, and current body mass index
        Twinship 0.76 (0.49–1.16)  
        Singleton Referent  
        Monozygote twin 0.90 (0.53–1.52)  
        Dizygote twin 0.58 (0.23–1.47)  
Cohort studies [64] 1980 LTCohort 270 (16,922) Denmark Twinship 1.1 (1.0–1.2) Observed/expected ratio (95% CI)
  [65] 1995 LTCohort 740 (25,541) Sweden Twinship 1.1 (1.0–1.1) Observed/expected ratio (95% CI)
        Monozygote twin 1.0 (0.9–1.2)  
        Dizygote twin 1.1 (1.0–1.2)  
  [67] 1999 LTCohort 245 (13,176) Finland Twinship 0.91 (0.81–1.00) Observed/expected ratio (95% CI)
        Monozygote twin 0.76 (0.59–0.97)  
        Dizygote twin 0.98 (0.84–1.10)  
  [66] 2000 Cohort 1,230 (29,197) USA Singleton Referent Adjusted for age, education, family history of breast cancer, age at menarche, age at first birth, height, current body mass index, body mass index at age 18, waist:hip ratio, alcohol drinking, and hormone replacement therapy
        Twinship 1.72 (1.22–2.42)  
        Singleton Referent  
        Monozygote twin 1.04 (0.43–2.5)  
        Dizygote twin 1.77 (1.16–2.7)  
  [31] 2006 Cohort 249 1,024 USA Singleton Referent Adjusted for age at diagnosis, diagnosis year, stage at diagnosis, and birth order, with exception of birth order, which is adjusted for maternal age
        Twinship 2.5 (1.0–6.2)  
  1. aAuthors used the female twins as the proxy of the monozygote twin and the female twin with male twin as the proxy of the dizygote twin. Cohort, cohort study; LCC, case-control study with linkage with population and cancer registry data; LTCohort, twin cohort study by using linkage with birth and cancer registry data; MCC, multicenter case-control study; NCC, nested case-control study in cohort; PCC, population-based case-control study.