The estimated effect of season and vitamin D in the first trimester on pubertal timing in girls and boys:a cohort study and an instrumental variable analysis

Background Season of birth has been associated with age at menarche. Maternal vitamin D levels in pregnancy may explain this effect. We investigated whether the season of first trimester or maternal 25-hydroxyvitamin D3 [25(OH)D3] levels were associated with pubertal timing in children. Methods We conducted a follow-up study of 15 819 children born in 2000–03 from the Puberty Cohort, nested in the Danish National Birth Cohort (DNBC). Mean differences in attaining numerous pubertal markers, including a combined estimate for the average age at attaining all pubertal markers, were estimated for low (November–April) relative to high (May–October) sunshine exposure season in the first trimester using multivariable interval-censored regression models. Moreover, we conducted a two-sample instrumental variable analysis using season as an instrument for maternal first-trimester 25(OH)D3 plasma levels obtained from a non-overlapping subset (n = 827) in the DNBC. Results For the combined estimate, girls and boys of mothers who had their first trimester during November–April had earlier pubertal timing than girls and boys of mothers whose first trimester occurred during May–October: –1.0 months (95% CI: –1.7 to –0.3) and –0.7 months (95% CI: –1.4 to –0.1), respectively. In the instrumental variable analysis, girls and boys also had earlier pubertal timing: respectively, –1.3 months (95% CI: –2.1 to –0.4) and –1.0 months (95% CI: –1.8 to –0.2) per SD (22 nmol/L) decrease in 25(OH)D3. Conclusions Both first pregnancy trimester during November–April and lower 25(OH)D3 were associated with earlier pubertal timing in girls and boys.