A regular pattern of ovulation, corresponding to achievement of fertility, usually develops rapidly once a girl begins having menstrual periods. However, girls who have a later onset of menstruation (after age 13) tend to have lower rates of regular ovulation in the years following the onset of menstruation. Studies have shown that one-half of adolescent girls who first begin to menstruate after age 13 will not ovulate regularly over the next four and a half years.