TABLE 3.

Association of frequency of night shifts per month and menstrual‐related variables in women working two rotating shifts

	Frequency of night shifts per month	Model 1		Model 2
		PR	95% CI	PR	95% CI
Irregular menstrual cycles	None	Reference		Reference
	1‐3 nights	1.27	0.92‐1.74	1.28	0.91‐1.80
	4‐5 nights	1.40	1.12‐1.75	1.41	1.08‐1.83
	≥6 nights	1.72	1.34‐2.20	1.78	1.34‐2.35
Amenorrhea ≥3 mo	None	Reference		Reference
	1‐3 nights	0.81	0.26‐2.50	0.75	0.26‐2.22
	4‐5 nights	1.37	0.70‐2.66	1.20	0.62‐2.31
	≥6 nights	2.58	1.32‐5.03	2.39	1.27‐4.50
Abnormal menstrual period	None	Reference		Reference
	1‐3 nights	0.87	0.41‐1.86	0.73	0.33‐1.61
	4‐5 nights	0.87	0.50‐1.49	0.69	0.38‐1.23
	≥6 nights	0.97	0.51‐1.85	0.81	0.41‐1.60
Dysmenorrhea or premenstrual symptoms affected their work	None	Reference		Reference
	1‐3 nights	1.01	0.87‐1.18	1.04	0.88‐1.23
	4‐5 nights	1.07	0.97‐1.19	1.11	0.98‐1.26
	≥6 nights	1.04	0.91‐1.19	1.09	0.94‐1.27
Often affected	None	Reference		Reference
	1‐3 nights	1.09	0.63‐1.88	1.15	0.66‐2.00
	4‐5 nights	1.27	0.88‐1.84	1.34	0.90‐2.00
	≥6 nights	1.26	0.80‐2.00	1.33	0.81‐2.16

Model 1: adjusted for age. Model 2: adjusted for age, body mass index, hospital size, and the department in which the women worked.

Abbreviations: CI, confidence interval; PR, prevalence ratio.