Menstrual bleeding patterns in untreated women. Task Force on Long-Acting Systemic Agents for Fertility Regulation.

Menstrual histories recorded by more than 1000 healthy, untreated women have been analyzed using the reference period method. Results were obtained for each year of age from 15 to 49. Between menarche and age 19, the most important feature of menstrual patterns is their variability. Within-woman mean segment (cycle) length then decreases slowly but steadily, from 29.0 days at age 20 to 26. 7 days at age 40. The range of segment lengths over a year also falls, to a minimum of 7 days at age 38. During the 40s, mean segment length rises to 29 days at age 49, and there is a sharp increase in the range, to 28 days. The length and variability of bleeding episodes, however, changes little between the ages of 19 and 49. Modifications to the World Health Organization definitions of 'clinically important' bleeding patterns are proposed.