Low digit ratio predicts early age at menarche in Colombian schoolgirls.

BACKGROUND The ratio between the lengths of the second and fourth fingers (digit ratio: 2D : 4D), a purported negative correlate of prenatal androgen exposure, has been inversely related to age at menarche. However, a recent study found high digit ratios in carriers of a single variant in the LIN28B gene, which has been linked to delayed menarche. METHODS We investigated the association of digit ratio and age at menarche in 299 pre-menarcheal girls aged 5-12 years who participated in a longitudinal cohort study in Bogotá, Colombia. Finger lengths were measured at baseline and the occurrence of menarche was periodically ascertained over a median 32 months of follow-up. We used time-to-event analysis to estimate median ages at menarche as well as hazard ratios for menarche according to tertiles of the digit ratio for each hand. RESULTS Estimated median age at menarche was lower for girls in the lowest digit ratio tertile of the right hand compared with those in the highest (12.0 vs. 12.3 years; P-value = 0.04). After adjustment for baseline age, height- and body mass index-for-age z-scores, the hazard of menarche was 86% higher in girls of the lowest digit ratio tertile (hazard ratio 1.9 [95% confidence interval 1.2, 2.9]) compared with those in the highest digit ratio tertile of the right hand. No significant associations were found with the left hand. CONCLUSIONS Digit ratio was positively associated with age at menarche in this longitudinal investigation, consistent with results from a recent gene-linkage study.