Table 2.

Menstrual Cycle Characteristics and Risk of Early Menopause in the NHS2 (1989–2011): Results of Cox Models

	Case Subjects	PY	HR	95% CI	P LRT a	P trend b	Adjusted HRc	95% CI	P LRT a	P trend b
Age at menarche, y					0.006	0.05			0.002	0.05
≤9	61	24,636	1.30	1.00 to 1.68			1.28	0.99 to 1.67
10	199	85,691	1.19	1.02 to 1.39			1.19	1.02 to 1.39
11	477	246,713	1.02	0.91 to 1.34			1.01	0.90 to 1.13
12	872	465,002	1	REF			1	REF
13	726	421,669	0.92	0.84 to 1.02			0.91	0.82 to 1.00
14	263	163,279	0.89	0.77 to 1.02			0.87	0.75 to 1.00
15	103	68,322	0.84	0.68 to 1.03			0.81	0.66 to 0.99
16	69	40,106	0.97	0.76 to 1.24			0.92	0.72 to 1.18
≥17	24	11,798	1.20	0.80 to 1.80			1.09	0.72 to 1.64
Y to regularity					<0.0001	<0.0001			<0.0001	<0.0001
≤1	1,394	695,404	1	REF			1	REF
1–2	773	380,813	1.04	0.95 to 1.14			1.03	0.95 to 1.13
3–4	174	110,130	0.83	0.71 to 0.97			0.80	0.69 to 0.94
≥5	271	176,292	0.75	0.66 to 0.86			0.73	0.64 to 0.83
Never	182	164,577	0.63	0.54 to 0.73			0.61	0.52 to 0.72
Cycle regularity in HS					<0.0001	<0.0001			<0.0001	<0.0001
Very regular	1,092	516,455	1	REF			1	REF
Regular	958	484,072	0.95	0.87 to 1.04			0.95	0.87 to 1.04
Usually irregular	322	224,071	0.71	0.63 to 0.81			0.70	0.62 to 0.80
Always irregular/no periods	196	178,415	0.57	0.49 to 0.66			0.55	0.48 to 0.64
Cycle regularity at age 18–22 y					<0.0001	<0.0001			<0.0001	<0.0001
Very regular	647	310,844	1	REF			1	REF
Regular	457	260,288	0.95	0.88 to 1.04			0.95	0.88 to 1.04
Usually irregular	142	116,274	0.65	0.57 to 0.74			0.65	0.57 to 0.74
Always irregular/no periods	76	76,687	0.52	0.44 to 0.61			0.51	0.43 to 0.60
Cycle length at age 18–22 y, d					<0.0001	<0.0001			<0.0001	<0.0001
<25	231	68,700	1.77	1.53 to 2.05			1.70	1.47 to 1.96
26–31	937	498,311	1	REF			1	REF
32–39	120	139,263	0.48	0.40 to 0.58			0.49	0.40 to 0.59
≥40	56	70,502	0.45	0.34 to 0.59			0.44	0.34 to 0.58

Models included 2794 events and 1,527,216 person-y of observation. Women reporting OC use in high school and/ or at age 18 y were dropped from models of cycle length and regularity.

Abbreviations: HS, high school; LRT, likelihood ratio test; PY, person-y REF, referent.

^a P values from LRT restricted to those with information on the main menstrual characteristic for each model.

^b P for trend from models with each variable as ordinal categorical with risk.

^cAdjusted model includes smoking (categories: never, past, and current status, and ≤14, 15–24, or ≥25 cigarettes per d); alcohol consumption (0, 0.1–10, >10–30, >30 g/d); parity (0, 1–2, ≥3); duration of breastfeeding (0, 1–3, >3–6, >6–12, >12–18, >18–24, >24–36, and >36 months); percentage of calories from vegetable protein (quintiles); BMI (American Dietary Association categories); dairy and supplemental sources of vitamin D (quintiles).