The average length of your menstrual cycle, defined as the first day of menstrual bleeding to the first day of the next menstrual period, is 28 days. But the University of Michigan Health System says that the length of a cycle can vary from 21 to 35 or even 45 days. Birth control pills or patches, which release hormones, can "reset" your menstrual cycle and change the timing of your monthly period. Other activities or circumstances may have the same results, though it may take several months or years to see results.
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Lose or Gain Weight
Losing a lot of weight or gaining a lot of weight will change your menstrual cycles. Weight gain or loss affects hormone levels, according to the University of Michigan Health System. Women who are anorexic or underweight due to illness may cease having periods altogether until their weight returns to normal. Weight loss or gain most often affects menstrual cycles when the weight changes is sudden and drastic, according to Penn State's Milton S. Hershey College of Medicine.
Strenuous exercise can also alter the timing of your period. Some female athletes cease menstruating altogether. Sophie Kennedy, writing for Vanderbilt University, says that strenuous exercise reduces the signals from the pituitary gland that control the release of hormones that regulate menstrual cycles. Prolonged cessation of periods can lead to bone loss.
Live With a Group of Women
In 1971, researcher Martha McClintock of the University of Chicago's Department of Psychology first reported on the idea of menstrual synchronicity, in which groups of women living in close proximity had roughly the same menstrual cycle. McClintock theorized that pheromones were responsible for this syncing of the women's cycles. McClintock reported on the phenomenon again in 1978. Since then, researchers have observed this synchronicity in lemurs and rats, but the cause of it hasn't been identified.