Age at menarche and age at first sexual intercourse: a prospective cohort study.

OBJECTIVE Younger age at menarche (AAM) may put girls at risk for earlier first sexual intercourse (FSI). Young age at FSI has far-reaching negative outcomes. We describe the longitudinal relationship between AAM and FSI in a large prospective birth cohort. METHODS AAM was collected from 554 girls from the Western Australia (Raine) Pregnancy Cohort Study, prospectively from age 10 or retrospectively at age 14. Age at FSI was collected at ages 17 and 20. Cox regression models describe likelihood of FSI by age and years since menarche for younger (<12 years) and older (≥14 years) AAM relative to average AAM (12-13 years). RESULTS Girls with younger AAM and average AAM were equally likely to have FSI by age 16 (adjusted hazard ratio [aHR]: 0.90 [95% confidence interval (CI): 0.60-1.35]). FSI by age 16 was less likely among girls with older AAM than those with average AAM (aHR: 0.35 [95% CI: 0.17-0.72]). Girls with younger AAM had a longer median interval between menarche and FSI than girls with average AAM (5.0 years [interquartile range: 4.4-8.5 years] vs 3.7 years [interquartile range: 2.4-5.3 years]). Those with younger AAM were less likely to report FSI within 4 years of menarche than those with average AAM (0-2 years aHR: 0.04 [95% CI: 0.01-0.31]; 2-4 years aHR: 0.36 [95% CI: 0.23-0.55]). By age 20, 429 girls (77.4%) reported FSI. CONCLUSIONS Younger AAM was not a risk factor for younger age at FSI in this cohort.