prolonged intervals between menstrual cycles. menstruation is the normal cyclic bleeding from the female reproductive (reproductive system disease) tract. Most women of reproductive age menstruate every 25 to 30 days if they are not pregnant, nursing a child, or experiencing other disorders such as tumours, anorexia nervosa, or Stein-Leventhal syndrome (polycystic ovary syndrome). In oligomenorrhea, menstruation occurs in intervals greater than 35 days. The cause of the disorder may be psychological, hormonal, or structural in nature. See also amenorrhea.