Woman with polymenorrhoea have shorter menstrual cycles when menstruation occurs in less than 21 days (the average menstrual cycle lasts 28 days). 

Patophysiology is unknown, but some scientists believe that it is because of accelerated follicular phase of menstrual cycle which leads to earlier onset of next menstruation. This could be caused by disruption in hypothalamic–pituitary–gonadal axis. It is caused by higher activation of anterior pituitary gland, which produces luteinizing hormone and prolactine.

Polymenorrhoea occures more frequently in adolescent and perimenopausal women. Sometimes shorten menstrual cycles follow after delivery of child when the pitutary gland is still more active.

Polymenorrhoea can be associated with anovulatory cycles, pelvic inflammatory diseases, endometriosis and also with polycystic ovary syndrome.

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Sources

Menstrual cycle ―sourced from Wikipedia licensed under CC BY- SA 3.0
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