Table 2. Summary risk estimates of the association between parity and colorectal cancer risk.

	No. of studies	Summary RR (95% CIs)	Q Statistic	I 2 Value (%)	P h *	P h **
All studies	9	0.95 (0.88–1.02)	9.04	0.5	0.433	–
Subgroup analyses						0.434
High quality studies (scores ≥8)	5	0.97 (0.89–1.06)	3.52	0	0.475
Low quality studies (scores <8)	4	0.91 (0.81–1.02)	4.82	16.9	0.307
Duration of follow-up						0.326
<10 y	5	0.98 (0.89–1.07)	5.01	20.1	0.287
≥10 y	4	0.91 (0.82–1.01)	2.94	0	0.568
Number of cases						0.980
<500	4	0.95 (0.77–1.17)	5.53	45.7	0.137
≥500	5	0.95 (0.88–1.02)	3.52	0	0.621
Geographic location						0.146
America	5	1.00 (0.90–1.12)	5	20	0.287
Europe	2	0.92 (0.84–1.01)	0.80	0	0.670
Asia	2	0.80 (0.59–1.09)	0.59	0	0.443
Anatomic cancer site						0.650
Colon	5	0.92 (0.80–1.05)	5.35	25.2	0.253
Rectum	3	0.86 (0.71–1.05)	1.59	0	0.451
Cancer subsite of colon						0.611
Proximal colon	3	0.87 (0.68–1.10)	0.42	0	0.810
Distal colon	3	0.95 (0.76–1.18)	1.38	0	0.501
Adjustment for confounders or important risk factors
Body mass index						0.434
Yes	5	0.97 (0.89–1.06)	3.52	0	0.475
No	4	0.91 (0.82–1.02)	4.82	16.9	0.307
Diabetes mellitus						0.498
Yes	2	0.97 (0.88–1.07)	0.04	0	0.834
No	7	0.92 (0.83–1.02)	8.46	17.3	0.293
Physical activity						0.808
Yes	4	0.95 (0.87–1.05)	1.97	0	0.579
No	5	0.94 (0.84–1.04)	7.00	28.6	0.220
Cigarette smoking						0.434
Yes	5	0.97 (0.89–1.06)	3.52	0	0.475
No	4	0.91 (0.82–1.02)	4.82	16.9	0.307
Alcohol drinking						0.808
Yes	4	0.95 (0.87–1.05)	1.97	0	0.579
No	5	0.94 (0.84–1.04)	7.00	28.6	0.220
HRT use						0.274
Yes	2	1.01 (0.89–1.14)	1.12	10.6	0.290
No	7	0.92 (0.85–1.00)	6.55	0	0.478
Family history of colorectal cancer/adenomatous polyposis						0.944
Yes	3	0.95 (0.84–1.07)	1.96	0	0.375
No	6	0.95 (0.87–1.03)	7.08	15.3	0.314

RR: relative risk; CI: confidence interval; HRT: hormone replacement therapy.

^* P value for heterogeneity within each subgroup.

^** P value for heterogeneity between subgroups with meta-regression analysis.