Associations between the number of children, age at childbirths and prevalence of chronic low back pain: the Nord-Trøndelag Health Study

Table 5 Prevalence of chronic LBP by reproductive factors in different strata of age

	PR (95% CI) for 1 childbirth vs. no childbirth^a	PR (95% CI) per child, among women with ≥1 childbirth^b	PR (95% CI) for age at first childbirth ≤19 vs. 25–29, among women with ≥2 childbirths^c	PR (95% CI) for age at first childbirth ≥35 vs. 25–29, among women with ≥2 childbirths^c
Age when information was collected (years)
20–29	1.32 (1.07–1.62)	1.19 (1.06–1.33)	1.49 (0.77–2.90)
30–39	1.15 (0.91–1.44)	0.99 (0.93–1.05)	1.27 (1.05–1.53)	1.12 (0.33–3.83)
40–49	0.99 (0.81–1.20)	1.04 (1.00–1.09)	1.46 (1.24–1.73)	1.42 (0.89–2.27)
50–59	0.99 (0.79–1.25)	1.00 (0.97–1.04)	1.46 (1.24–1.72)	1.47 (0.89–2.43)
60–69	0.81 (0.62–1.05)	0.95 (0.91–0.98)	1.21 (0.99–1.48)	0.82 (0.42–1.62)
P for interaction with categorical age	0.020	0.002	0.10^d
P for interaction with linear age	0.001	0.002	0.75^d

^aAdjustment for BMI, physical activity, education, smoking
^bAdjustment for BMI, physical activity, education, smoking, age at first childbirth
^cAdjustment for BMI, physical activity, education, smoking, number of childbirths, age at last childbirth
^dBased on likelihood ratio test for interaction with complete quadratic relationship with age at first childbirth

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