World Health Organization multicenter study on menstrual and ovulatory patterns in adolescent girls. II. Longitudinal study of menstrual patterns in the early postmenarcheal period, duration of bleeding episodes and menstrual cycles. World Health Organization Task Force on Adolescent Reproductive Health.

During a two-year study, 670 girls (aged 11-15 years) submitted menstrual diaries that started at the onset of their first menstrual bleeding (Group I), and 802 girls (aged 11-15 years) who had already experienced menarche prior to the study (Group II) kept similar diaries. The two groups were recruited in three centers: Colombo (urban), Peradeniya (rural), both in Sri Lanka, and Hong Kong. The mean duration of bleeding episodes was 4.7 days (SD 1.8) in Group I and 4.9 days (SD 1.4) in Group II. The median length of the first cycle after menarche was 34 days, with 38.3% of the cycles being longer than 40 days. By the 5th cycle the median length was 31 days, similar to that of Group II. The establishment of cycle regularity was defined by three successive cycles within the range of 10 days with none of the three cycles shorter than 20 or longer than 40 days. This regularity was achieved by 19% of the Group I girls in the first three cycles and by 67% of these girls during the entire study period, compared to 89% in Group II.