Regular periods involve ovulatory cycle & endometrial cycle making 28 days interval with 4-5 days menstruation.
At PCOS , OVULATON(Rupture of egg follicles) may disturbed. There may be failure of ovulation, immature ovulation, or late ovulation.
Bleeding may be very irregular. It might be 2-3 months once, or 15-20 days once. Even there may be absence of menstrual cycle for 6-12months also. Sometimes heavy bleeding, alternate with spotting will be present. Flow may prolong days together.
Dysmenorrhea (painful periods) will also be a symptom of PCOS.