Table 5. Summary risk estimates of the association between parity and endometrial cancer risk (parity number of 1 versus nulliparous).

	No of reports	RR (95% CI)	I2 (%)	P for heterogeneity
Parity number of 1 vs. nulliparous	19	0.73 (0.64–0.84)	88.3	<0.001
Subgroup analysis
Study design
Prospective	4	0.54 (0.40–0.72)	74.7	0.008
Case-control	14	0.83 (0.76–0.91)	35.2	0.093
Location
Europe	9	0.70 (0.53–0.91)	92.8	<0.001
America	5	0.85 (0.77–0.93)	0.0	0.494
Asia	4	0.43 (0.29–0.63)	6.1	0.362
International	1	0.88 (0.84–0.92)	—	—
Number of cases
<200	6	0.79 (0.64–0.97)	41.4	0.129
≥200	13	0.71 (0.60–0.84)	91.7	<0.001
Study publication time
Earlier than 1992	7	0.81 (0.74–0.89)	0.0	0.782
1992–	12	0.66 (0.54–0.81)	92.7	<0.001
Estimate adjustment
Yes	17	0.69 (0.58–0.82)	87.6	<0.001
No	2	0.89 (0.82–0.96)	5.7	0.303
Estimate adjusted for age
Yes	17	0.69 (0.58–0.82)	87.6	<0.001
No	2	0.89 (0.82–0.96)	5.7	0.303
Estimate adjusted for BMI
Yes	4	0.66 (0.43–1.01)	56.0	0.078
No	15	0.74 (0.63–0.86)	90.4	<0.001
Estimate adjusted for smoking
Yes	3	0.86 (0.69–1.06)	0.0	0.496
No	16	0.72 (0.62–0.84)	90.1	<0.001
Estimate adjusted for age, BMI and smoking
Yes	3	0.86 (0.69–1.06)	0.0	0.496
No	16	0.72 (0.62–0.84)	90.1	<0.001
Study quality
high	15	0.67 (0.55–0.80)	83.2	<0.001
low	3	0.92 (0.85–0.99)	0.0	0.424