Risk of uterine leiomyomata with menstrual and reproductive factors in premenopausal women: Korea nurses’ health study

Table 3 Associations of reproductive history and oral contraceptive use with the risk of uterine leiomyomata

Characteristics (N = 7,360)	Person-years	Cases	Age-adjusted HR (95% CI)	Multivariate HR (96% CI)
Parity^a
Nulliparous	22,047	327	Reference	Reference
Parous	10,025	120	0.44 (0.34–0.56)	0.40 (0.30–0.53)
No. of births
1	4214	37	Reference	Reference
2	5184	74	1.10 (0.72–1.68)	1.10 (0.72–1.68)
≥ 3	626	9	1.06 (0.50–2.23)	1.17 (0.55–2.48)
P for trend			0.737	0.625
Age at first birth, years
≤ 28	3178	51	Reference	Reference
29–30	2231	19	0.57 (0.33–0.96)	0.58 (0.34–0.98)
≥ 31	3217	37	0.69 (0.45–1.06)	0.67 (0.44–1.03)
P for trend			0.113	0.086
Total breastfeeding, months
0	837	17	Reference	Reference
1–5	3058	38	0.71 (0.40–1.26)	0.64 (0.36–1.14)
6–11	1849	25	0.83 (0.45–1.56)	0.80 (0.42–1.50)
≥ 12	2169	23	0.69 (0.36–1.32)	0.63 (0.33–1.21)
P for trend			0.472	0.415
Attempting to conceive (≥ 1 year)^b
No	30,186	428	Reference	Reference
Yes	1886	19	0.62 (0.39–0.98)	0.70 (0.44–1.12)
Consulting a physician
No	957	9	Reference	Reference
Yes	929	10	1.05 (0.42–2.59)	1.36 (0.52–3.54)
Oral contraceptive use (≥ 2 months)^b
No	29,874	422	Reference	Reference
Yes	2195	25	0.88 (0.59–1.33)	0.98 (0.65–1.48)
Age at first use, years
≤ 26	1323	15	Reference	Reference
> 26	752	9	0.96 (0.34–2.72)	0.75 (0.25–2.25)

^aAdjusted for age at baseline, age at menarche, menstrual cycle length, body mass index, systolic blood pressure, and a recent gynecologic examination
^bAdjusted for age at baseline, age at menarche, menstrual cycle length, parity history, body mass index, systolic blood pressure, and a recent gynecologic examination
CI, confidence interval; HR, hazard ratio

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