Table 1 Demographic characteristics of study population.

Characteristics	Controls (N = 941)		Cases (N = 959)		p-value^a
Characteristics	N	%	N	%	p-value^a
Age group (years)					0.538
≤29	21	2.24	22	2.29
30–39	102	10.84	124	12.93
40–49	424	45.06	444	46.30
50–59	293	31.14	288	30.03
60–69	91	9.67	75	7.82
≥70	10	1.06	6	0.63
Number of pregnancies					<0.001
Nullipara	96	10.20	125	13.03
1–2	279	29.65	327	34.10
≥3	482	51.22	503	52.45
Age at birth of first child (years)					<0.001
Nullipara	96	10.20	125	13.03
<28	481	51.12	461	48.07
≥28	252	26.78	341	35.56
Education					0.902
<High school	359	38.15	372	38.79
≥University	416	44.21	425	44.32
BMI (kg/m²)					<0.001
<18.5	671	71.31	677	70.59
≥18.5–24.9	53	5.63	39	4.07
≥25	151	16.05	226	23.57
Age at menarche (years)					<0.001
≤14	558	59.30	681	71.02
>14	311	33.05	276	28.78
Tobacco smoking					<0.001
Never	804	85.44	846	88.22
Ever	78	8.29	113	11.78
Alcohol drinking					<0.001
Never	476	50.58	681	71.01
Ever	406	43.15	278	28.99
Use of female hormone treatment in menopausal women^b					0.197
Never	239	25.40	278	28.99
Ever	81	8.61	74	7.72
Family history of breast cancer in first degree relatives					<0.001
No	745	79.17	706	73.62
Yes	129	13.71	231	24.09
Menopausal status					<0.001
Premenopause	416	44.21	575	59.96
Postmenopause	525	55.79	384	40.04
Night shift work experience					0.497
Never	862	91.60	870	90.72
Ever	79	8.40	89	9.28
Duration of night shift work					0.459
Never	862	90.72	870	90.72
<2 years	33	3.51	32	3.34
≥2 years	43	4.57	56	5.84
Unknown	3	0.32	1	0.10
Estrogen receptor
Negative	—		338	35.25
Positive	—		586	61.11
Unknown	—		35	3.65
Progesteron receptor
Negative	—		400	41.71
Positive	—		525	54.74
Unknown	—		34	3.55
Human epidermal growth factor receptor 2
Negative	—		697	72.68
Positive	—		173	18.04
Unknown	—		89	9.28

N: number; BMI: body mass index (kg/m²); ^ap value of chi-square test; ^bin postmenopausal women.

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