It has been established that a womans menstrual cycle experiences changes on a regular basis. Experts say that alterations in ones cycle takes place every three months (on an average). What does this mean exactly? It means your period will not start on the date its supposed to. It might start earlier or later. However, theres not to be alarmed about as this is a normal cycle, and part of your bodys changes.In case youve noticed that your period is constantly off balance month on month, that it doesnt start on the same date its supposed to, or at times, you may not get it at all, theres a chance you have an irregular cycle. Your period is considered irregular if the duration between your last period and your next period is 38 days or it constantly varies.So what is the reason behind an irregular cycle? Here are some aspects to consider:Hormonal Birth ControlAn irregular cycle could be a side-effect of hormonal birth control pills. These and even hormone-containing intrauterine devices (IUDs) can cause irregular bleeding. If thats not all, birth control pills may cause spotting between periods, and result in much lighter periods while IUDs may cause heavy bleeding.PerimenopauseFor older women, an irregular cycle may be termed as perimenopause, or the transition phase before entering menopause. While it usually begins in your 40s, it can set in earlier for some women.Typically, you may experience signs and symptoms lasting from four to eight years, and it begins with changes to your menstrual cycle. The fluctuating levels of oestrogen during perimenopause may cause your menstrual cycles to get longer or shorter.Thyroid DisordersAccording to the 2015 study titled Role of Thyroid Dysfunction in Patients with Menstrual Disorders in Tertiary Care Center of Walled City of Delhi published in 2015 in The Journal of Obstetrics and Gynecology of India, 44 per cent of participants with menstrual irregularities were also diagnosed with thyroid disorders. Hypothyroidism, or an underactive thyroid, can cause longer, heavier periods and increased cramping. You may also experience fatigue, sensitivity to cold, and weight gain. On the other hand, hyperthyroidism, or an overactive thyroid, can cause shorter, lighter periods. A person diagnosed may also experience sudden weight loss, anxiety and nervousness, and heart palpitations.Polycystic Ovary Syndrome (PCOS)Irregular periods are the most common side-effect of PCOS. If you have been diagnosed with PCOS, theres a good chance that you may even miss your period and then experience heavy bleeding when you get them.Being OverweightUnnatural or rapid weight gain and even obesity is known to result in menstrual irregularity. In fact, research into the subject has revealed that being overweight tends to impact the normal functioning of hormones and insulin levels, which tend to interfere with your menstrual cycle.Extreme Weight LossExcessive or rapid weight loss, or even eating disorders, can cause your period to stop. A severe calorie deficit can interfere with the production of the hormones needed for ovulation. If your body mass index (BMI) is lower than 18.5, you are considered underweight. Apart from a halted period, you may also experience fatigue, headaches, and hair loss.