Women who have menstrual irregularity are most often diagnosed with PCOS because there is a link, but as already said the link is not absolute: you might have PCOS and still have regular periods, and you might have irregular periods and are not a PCOS sufferer complicated as that Statistics says approximately 5% of women in reproductive age are diagnosed with PCOS and approximately 70% have irregular periods as a symptoms of PCOS. It is very interesting to know that there is no common definition of PCOS: The Rotterdam Consensus Criteria (2006) requires two of the three signs/symptoms of PCOS, and the signs include: hyperandrogenism, irregular menstrual periods, polycystic-appearing ovaries on pelvic ultrasound to be present for the diagnosis to be made.
The Androgen Excess Society (2006) on the other hand requires hyperandrogenism plus one of the other two signs/symptoms (irregular menstrual periods, polycystic-appearing ovaries on pelvic ultrasound). There is differential diagnosis of someone with irregular menstrual periods available, and these include both hypothyroidism, hyperthyroidism, Prolactin/Pituitary disorders, Nonclassical congenital adrenal hyperplasia (Nonclassical CAH), Androgen-secreting tumor (ovary, adrenal gland), Exogenous androgens, Primary hypothalamic amenorrhea (stress-related, exercise-related, eating disorders, low body weight), Central nervous system tumors/disorders, Primary ovarian failure, Cushing syndrome, Insulin-receptor defects.
Women are often placed on birth control pills that regulate menstrual cycles. I believe this is not the best option not because contraceptives themselves but because the result is cured but the cause of the irregular bleeding might still be there although the bleeding is regulated . Therefore it is essential that the health condition of a woman with irregular periods is evaluated before diagnosis is made although the diagnosis such as PCOS might seem obvious. This is essential because proper evaluation and diagnosis as said above also affects proper treatment.