breast-cancer-research.com

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.
Park et al. Breast Cancer Research 2008 10:R8 doi:10.1186/bcr1850

Table 6