Table 3.

Associations between age at menarche and risk of chronic LBP in HUNT3 in prospective analysis

	Total number of women in category of age at menarche*	Number of women with chronic LBP	Crude AR of chronic LBP (%)	RR (95% CI) with adjustment for age only	RR (95 % CI) with additional adjustment†
Number of women included			11 659	11 659	10 854
Age at menarche (year)
≤11	930	236	25.4	1.34 (1.17 to 1.53)	1.32 (1.15 to 1.52)
12	2228	468	21.0	1.11 (1.00 to 1.24)	1.12 (1.00 to 1.25)
13	3340	641	19.2	1.01 (0.91 to 1.12)	1.01 (0.91 to 1.13)
14	3014	582	19.3	1.00 (reference)	1.00 (reference)
15	1600	314	19.6	1.01 (0.89 to 1.14)	1.02 (0.90 to 1.16)
16	421	82	19.5	1.00 (0.81 to 1.23)	1.01 (0.81 to 1.25)
≥17	126	30	23.8	1.24 (0.90 to 1.70)	1.43 (1.04 to 1.98)
P for categorical effect				0.001	0.002
P for linear trend				0.001	0.008
P for quadratic effect‡				0.001	<0.001

*In analysis adjusted for age only.

†Adjustment for age, BMI, physical activity, education, smoking, nulliparity and age at first delivery.

‡In model also including linear effect.

AR, absolute risk; BMI, body mass index; HUNT, Trøndelag Health Study; LBP, low back pain; RR, relative risk.