Odds ratio (95% CI) of early age at menarche (≤13 years old) for childhood passive smoking exposure in women who have never smoked.

Model A: Unadjusted (n = 20,061). Model B: Adjusted for age (n = 20,061). Model C: Adjusted for age and education of the participants (n = 20,061). Model D: Adjusted for age and education, childhood family economic status, childhood family social status, frequency of eating meat and hunger as a child (phase 3 only; n = 6,697). The number of smokers and the frequency of exposure were included in the model separately. Odds ratio (95% CI) of early age at menarche (≤13 years old) for childhood passive smoking exposure in women who have never smoked.