Why Are My Periods Irregular? The Most Common Causes Explained

The word 'irregular' gets used a lot around periods - but what does it actually mean? A regular menstrual cycle can be anywhere from 21 to 35 days, and that cycle can vary by a few days from month to month and still be completely normal. Irregular periods are those that consistently fall outside this range, or that change significantly from one cycle to the next without an obvious cause.

PCOS: The Most Common Cause

Polycystic ovary syndrome (PCOS) is the most common cause of irregular periods in women of reproductive age, affecting up to 1 in 5 Indian women. In PCOS, hormonal imbalance disrupts ovulation - so periods may be infrequent, unpredictable, or absent for months at a time. If your periods are consistently irregular and you have other symptoms such as excess hair growth, acne, or weight gain around the abdomen, PCOS is worth investigating.

Thyroid Disorders

The thyroid gland regulates metabolism, and it also interacts closely with the hormonal systems that control the menstrual cycle. Both an underactive thyroid (hypothyroidism) and an overactive thyroid (hyperthyroidism) can cause menstrual irregularity - with hypothyroidism more commonly causing heavy, irregular periods, and hyperthyroidism more often causing very light or absent periods.

A simple blood test (TSH) can identify thyroid dysfunction. If it is found, thyroid medication* can effectively regulate the condition and often brings the cycle back to normal. Thyroid problems are significantly more common in women than in men, and Indian women are particularly affected.

Stress and the HPA Axis

Chronic stress - whether physical, emotional, or nutritional - can disrupt the hormonal cascade that triggers ovulation. When the body perceives it is under threat, it deprioritises reproduction. The result can be delayed, light, or absent periods. This is why major life events, exam periods, or significant weight loss can all throw the cycle off.

If stress is the cause, the solution sounds simple but is genuinely difficult: reduce the stressor, improve sleep, and give the body adequate nutrition. If the cycle does not return within a few months, further investigation is warranted.

Significant Weight Change

Both significant weight loss and rapid weight gain can disrupt the menstrual cycle. Oestrogen is partly produced in fat tissue, so very low body fat - common in athletes, women with eating disorders, or those who have dieted aggressively - can reduce oestrogen to the point where periods stop. This is not a benign state: it is associated with bone density loss and other health consequences.

Perimenopause

For women in their 40s, irregular periods are often the first sign of perimenopause. As the ovaries begin to produce less oestrogen, ovulation becomes less regular. Periods may become shorter or longer, lighter or heavier, and gaps between them may widen. If you are in your mid-to-late 40s and noticing changes in your cycle alongside other symptoms - sleep disruption, mood shifts, or hot flushes - perimenopause is a strong possibility.

Other Causes Worth Knowing

Other reasons for irregular periods include elevated prolactin levels (which can be caused by a benign pituitary tumour or certain medications*), primary ovarian insufficiency (when the ovaries stop functioning normally before age 40), and certain medications including some antipsychotics and antidepressants*. This is why investigation matters - 'just irregular' is never a complete answer.