Table 3.

Association between mean sun hours per day available in trimester 1 and 2 and the combined average exposure over trimester 1 and 2, within sibling pairs where one sibling is born term and the other preterm to the same mother within Scotland 2000–2010.

Available sun exposure	Sibling univariable preterm birth OR (95% CI)	p	Sibling multivariable preterm birth OR (95% CI) Model 1	p	Sibling multivariable preterm birth OR (95% CI) Model 2	p
Trimester 1	0.98 (0.96, 1.0)	0.02	0.98 (0.96, 1.0)	0.11	0.98 (0.95, 1.01)	0.19
Trimester 2	1.00 (0.98, 1.02)	0.78	1.01 (0.99, 1.03)	0.19	1.04 (1.01, 1.08)	<0.01
Average trimester 1+2	0.98 (0.91, 1.06)	0.74	1.0 (0.97, 1.02)	0.82	1.02 (0.97, 1.06)	0.34

Model 1 adjusted for alternate trimester exposure, maternal age, smoking status, SIMD category and parity; for exposure “average trimester 1+2” adjusted for maternal age, smoking status, SIMD category and parity.

Model 2 adjusted for alternate trimester exposure, season of conception, maternal age, smoking status, SIMD and parity; for exposure “average trimester 1+2” adjusted for season of conception, maternal age, smoking, SIMD category and parity.