Menstrual Irregularities can be caused by some of the following options:

Pregnancy or breastfeeding : A missed period can be an early sign of pregnancy. Breastfeeding typically delays the return of menstruation after pregnancy.
Eating disorders, extreme weight loss or excessive exercising : Eating disorders - such as anorexia nervosa - extreme weight loss and increased physical activity can disrupt menstruation.
Polycystic ovary syndrome (PCOS) : Women with this common endocrine system disorder(PCOS) may have irregular periods as well as enlarged ovaries that contain small collections of fluid - called follicles - located in each ovary as seen during an ultrasound exam.
Premature ovarian failure : Premature ovarian failure refers to the loss of normal ovarian function before age 40. Women who have a premature ovarian failure - also known as primary ovarian insufficiency - might have irregular or occasional periods for years.
Pelvic inflammatory disease (PID) : is an infection of the reproductive organs that cause irregular menstrual bleeding.
Uterine fibroids : Uterine fibroids are noncancerous growths of the uterus. They can cause heavy menstrual periods and prolonged menstrual periods.

The doctors usually perform the either one or a combination of the following diagnostic tests to pinpoint the cause of the menstrual irregularities:

Pap Smear : to check for various infections or cancerous cells.
Blood Tests : to check for anemia, blood-clotting problems, and thyroid function.
Pelvic Ultrasound : produce images of the patient’s uterus, ovaries, and pelvis.
Endometrial Biopsy : takes a sample of the patient’s uterine tissue for analysis.
Diagnostic hysteroscopy : to view the inside of the patient’s uterus. For a hysteroscopy, the doctor will use a lighted tube to view the uterus and remove the polyp.
Sonohysterogram : is an ultrasound where a fluid is injected into the patient’s to help make an image of the uterine cavity, which is done to look for polyps or fibroids.
Pregnancy Test

The doctor usually performs the irregular periods treatment depending on the issues that cause the menstruation problems :

Puberty and menopause : Irregular periods that occur during puberty or as the patient approaches menopause do not usually need treatment.
Birth control : If the irregular menstruation is due to the use of contraception, and it continues for several months, it is recommended that the patient approach the doctor for other options.
PCOS and Obesity : If the patient who is overweight, or obese starts losing weight, it might help stabilize menstruation. Once the patient has reduced her weight, the body need not produce so much insulin, leading to lower testosterone levels and a better chance of ovulating.
Thyroid problems : The doctor might recommend treatment for the underlying problem by recommending medication, radioactive iodine therapy or surgery.
Stress and eating disorders : The doctor might recommend psychological therapy if emotional stress, an eating disorder, or sudden weight loss have triggered irregular periods. This might include relaxation techniques, stress management, and talking to a therapist.