Table 1. Characteristics of studies of parity and colorectal cancer risk.
| First author, publication year (reference), Country, Study design | Cases/subject (age), duration of follow up | Parity categories (exposure/case assessment) | HR/RR (95% CI) | Matched/Adjusted factors |
| Zervoudakis et al [13], 2011, USA, CS | 2,014/214,162 (50–71 y), 8.2 y | CRC Ever parous vs. Nulliparous | 0.98 (0.86–1.12) | Age, BMI, education level, alcohol consumption, family history of colorectal cancer, race, smoking history, DM, PA, and use of hormone therapy |
| CRC≥5 vs. Nulliparous | 0.95 (0.79–1.14) | |||
| (Self-questionnaire/cancer registry) | ||||
| Tsilidis et al [14], 2010, European, CS | 1,878/337,802 (35–70 y), 9 y | CRC Ever parous vs. Nulliparous | 0.96 (0.83–1.10) | Participating center, age at recruitment, smoking status, DM, BMI, PA, and alcohol use |
| CRC≥4 vs. 1 | 1.17 (0.97–1.42) | |||
| (Self-questionnaire/cancer registry) | ||||
| Akhter et al [20], 2008, Japan, CS | 538/48,511 (40–69 y), 12 y | CRC Ever parous vs. Nulliparous | 0.86 (0.60–1.24) | Age, PHC area, family history of colorectal cancer, BMI, leisure time PA, cigarette smoking, and alcohol drinking |
| CRC≥3 vs. Nulliparous | 0.83 (0.57–1.19) | |||
| CC Ever parous vs. Nulliparous | 0.94 (0.59–1.49) | |||
| CC≥3 vs. Nulliparous | 0.92 (0.57–1.47) | |||
| Proximal CC Ever parous vs. Nulliparous | 0.83 (0.45–1.54) | |||
| Proximal CC≥3 vs. Nulliparous | 0.87 (0.47–1.62) | |||
| Distal CC Ever parous vs. Nulliparous | 1.09 (0.50–2.35) | |||
| Distal CC≥3 vs. Nulliparous | 0.99 (0.45–2.14) | |||
| RC Ever parous vs. Nulliparous | 0.71 (0.40–1.27) | |||
| RC≥3 vs. Nulliparous | 0.70 (0.39–1.25) | |||
| (Self-questionnaire/cancer registry) | ||||
| Kabat et al [19], 2008, Canada, CS | 1,142/89,835 (40–59 y), 16.4 y | CRC Ever parous vs. Nulliparous | 1.19 (0.85–1.66) | Age, BMI, menopausal status, pack-years of smoking, OC use, HRT use, education, age at menarche, and age at first live birth |
| CRC≥5 vs. Nulliparous | 1.18 (0.81–1.71) | |||
| CC≥5 vs. Nulliparous | 1.20 (0.77–1.86) | |||
| Proximal CC≥5 vs. Nulliparous | 1.79 (0.94–3.39) | |||
| Distal CC≥5 vs. Nulliparous | 0.79 (0.40–1.59) | |||
| RC≥5 vs. Nulliparous | 1.05 (0.54–2.05) | |||
| (Self-questionnaire/cancer registry) | ||||
| Lin et al [21], 2007, USA, CS | 267/39,680 (≥45 y), 11 y | CRC≥5 vs. Nulliparous | 0.84 (0.53–1.35) | Age, randomized treatment assignment, family history of colorectal cancer, previous history of benign colorectal polyps, BMI, PA, smoking status, red meat intake, alcohol consumption, baseline aspirin use, multivitamin use, baseline postmenopausal hormone use, and OC use |
| CC≥5 vs. Nulliparous | 0.79 (0.45–2.37) | |||
| RC≥5 vs. Nulliparous | 0.90 (0.33–2.47) | |||
| (Self-questionnaire/medical records) | ||||
| Tamakoshi et al [22], 2004,Japan, CS | 207/38,420 (40–79 y), 7.6 y | CC Ever parous vs. Nulliparous | 0.65 (0.35–1.20) | Age at baseline, study area, smoking status, alcohol drinking habit, exercise, meat intake, green leafy vegetable intake, family history of colon cancer, and BMI at baseline |
| CC≥4 vs. 1 | 0.90 (0.47–1.74) | |||
| (Self-questionnaire/cancer registry) | ||||
| Troisi et al [15], 1997, USA, CS | 203/57,529 (31–90 y), 10 y | CRC Ever parous vs. Nulliparous | 0.87 (0.66–1.16) | Age |
| CRC≥4 vs. Nulliparous | 1.00 (0.72–1.50) | |||
| CC Ever parous vs. Nulliparous | 0.87 (0.58–1.36) | |||
| CC≥4 vs. Nulliparous | 0.91 (0.54–1.50) | |||
| Proximal CC Ever parous vs. Nulliparous | 0.74 (0.42–1.31) | |||
| Proximal CC≥4 vs. Nulliparous | 0.67 (0.31–1.40) | |||
| Distal CC Ever parous vs. Nulliparous | 1.45 (0.65–3.23) | |||
| Distal CC≥4 vs. Nulliparous | 1.80 (0.74–4.50) | |||
| RC Ever parous vs. Nulliparous | 1.40 (0.58–3.39) | |||
| RC≥4 vs. Nulliparous | 1.60 (0.57–4.30) | |||
| (Self-questionnaire/medical records) | ||||
| Martínez et al [16], 1997, USA, CS | 501/89,448 (30–55 y), 12 y | CRC≥5 vs. 1 | 1.57 (1.02–2.41) | Age, BMI, PA, family history of colorectal cancer, aspirin use, cigarette smoking, alcohol consumption, intake of red meat, OC use, postmenopausal hormone use, age at menarche, age at first pregnancy, and age at menopause |
| CC≥5 vs. 1 | 1.57 (0.97–2.53) | |||
| RC≥5 vs. 1 | 1.63 (0.63–4.21) | |||
| (Self-questionnaire/medical records) | ||||
| Broeders et al [23], 1996, Sweden, NC-CS | 2,148/10,738 (20–59 y), 25 y | CC Ever parous vs. Nulliparous | 0.90 (0.77–1.06) | Age |
| CC≥5 vs. Nulliparous | 0.77 (0.54–1.10) | |||
| Proximal CC Ever parous vs. Nulliparous | 0.91 (0.68–1.22) | |||
| Proximal CC≥5 vs. Nulliparous | 0.76 (0.38–1.52) | |||
| Distal CC Ever parous vs. Nulliparous | 0.90 (0.71–1.15) | |||
| Distal CC≥5 vs. Nulliparous | 0.74 (0.44–1.26) | |||
| RC Ever parous vs. Nulliparous | 0.86 (0.69–1.06) | |||
| RC≥5 vs. Nulliparous | 0.99 (0.64–1.51) | |||
| (NA/cancer registry) | ||||
| Bostick et al [17], 1994, USA, CS | 212/35,215 (55–69 y), 5 y | CC Ever parous vs. Nulliparous | 1.63 (0.93–2.86) | Age, total energy intake, height, total vitamin E intake, a total vitamin E by age interaction term, and vitamin A supplement intake |
| CC≥3 vs. Nulliparous | 1.80 (1.02–3.19) | |||
| (Self-questionnaire/cancer registry) | ||||
| Wu et al [24], 1987, USA, CS | 68/11,888 (N/A), 4.5 y | CRC Ever parous vs. Nulliparous | 1.05 (0.63–1.76) | None |
| CRC≥3 vs. Nulliparous | 0.50 (0.20–1.30) | |||
| (Self-questionnaire/medical records) |
HR: hazards ratio; RR: relative risk; CRC: colorectal cancer; CC: colon cancer; RC: rectal cancer; CI: confidence interval; NC-CS: nested case-control study; CS: cohort study; N/A: not available; BMI: body mass index; OC: oral contraceptive; PA: physical activity; DM: diabetes mellitus.