Table 6.
Studies assessing the association of twinship with risk for breast cancer
| 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) | ||||||||
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.