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