Table 3.

Incidence and relative incidence of surgery for CM users and nonusers.

	CM nonusers			CM users			IRRb	(95% CI)
	Patients with surgery	Person-years	IRa	Patients with surgery	Person-years	IR
Overall	2074	19994	103.73	1607	90744	17.71	0.17	(0.16–0.18)***
Age, years
<20	0	144	0.00	0	843	0.00	—
20–29	66	1578	41.83	84	8849	9.49	0.23	(0.16–0.31)***
30–39	509	5390	94.43	636	26484	24.01	0.25	(0.23–0.29)***
40–49	1281	9488	135.01	807	41316	19.53	0.14	(0.13–0.16)***
≥50	218	3394	64.23	80	13252	6.04	0.09	(0.07–0.12)***
Income, US$ per month
<564.3	516	5401	95.54	406	24169	16.80	0.18	(0.15–0.20)***
564.3–656.5	415	3854	107.68	334	18075	18.48	0.17	(0.15–0.20)***
656.6–779.6	671	5268	127.37	469	26095	17.97	0.14	(0.13–0.16)***
≥779.7	472	5470	86.29	398	22406	17.76	0.21	(0.18–0.24)***
Occupational status
White collar	1094	11524	94.93	899	52095	17.26	0.18	(0.17–0.20)***
Blue collar	779	6078	128.17	559	29825	18.74	0.15	(0.13–0.16)***
Others	201	2392	84.03	149	8824	16.89	0.20	(0.16–0.25)***
Area
Northern Taiwan	946	11161	84.76	618	41349	14.95	0.18	(0.16–0.20)***
Central Taiwan	361	2137	168.93	368	17166	21.44	0.13	(0.11–0.15)***
Southern Taiwan	682	5759	118.42	549	28345	19.37	0.16	(0.15–0.18)***
Eastern Taiwan and offshore islands	85	935	90.91	72	3884	18.54	0.20	(0.15–0.28)***
Comorbidity
Excessive menstruation
No	1653	16616	99.48	1056	71005	14.87	0.15	(0.14–0.16)***
Yes	421	3378	124.63	551	19739	27.91	0.22	(0.20–0.25)***
Iron-deficiency anemia
No	1800	18054	99.70	1225	79864	15.34	0.15	(0.14–0.17)***
Yes	274	1940	141.24	382	10880	35.11	0.25	(0.21–0.29)***
Dysmenorrhea
No	1776	17481	101.60	1072	72183	14.85	0.15	(0.14–0.16)***
Yes	298	2512	118.63	535	18562	28.82	0.24	(0.21–0.28)***
Infertility
No	2072	19929	103.97	1592	90011	17.69	0.17	(0.16–0.18)***
Yes	2	65	30.77	15	733	20.46	0.67	(0.15–2.91)

^aIR: incidence rate per 1,000 people.

^bIRR: incidence rate ratio, compared to CM nonusers.

***P < 0.0001.