Irregular Menstrual Cycles

A normal menstrual cycle runs 21 to 35 days with reasonably consistent timing. Irregularity means cycles outside that range, varying by more than seven to nine days between cycles, or stopping altogether. The commonest cause in reproductive-age women is polycystic ovary syndrome, which accounts for the majority of chronic anovulation and typically produces long, infrequent cycles alongside acne, unwanted hair growth or difficulty losing weight. Thyroid disease — both over- and underactive — is the next most important, is easily tested, and is frequently missed. Hypothalamic amenorrhoea, where cycles stop because of low energy availability, is under-recognised and specifically affects athletes, those with restrictive eating patterns, and anyone in a substantial calorie deficit; it deserves emphasis because the treatment is eating more and training less, not hormonal suppression. Raised prolactin, often from a benign pituitary adenoma or medication, is another checkable cause. Cycles are also normally irregular in the first two years after menarche and in the perimenopausal transition, when variability is expected rather than pathological. Because chronic anovulation leaves the endometrium under unopposed oestrogen, prolonged irregularity carries endometrial cancer risk and is not something to simply monitor indefinitely.

• •Cycles shorter than 21 days or longer than 35 days
• •Variation of more than 7-9 days between consecutive cycles
• •Missed periods or complete absence of periods
• •Unpredictable bleeding or spotting between periods
• •Very heavy or very light bleeding
• •Difficulty conceiving due to absent or infrequent ovulation
• •Acne, unwanted facial hair or scalp thinning suggesting PCOS
• •Weight change, fatigue, hair loss or temperature intolerance suggesting thyroid disease
• •Milky nipple discharge suggesting raised prolactin
• •Hot flushes and night sweats in perimenopause

• •Polycystic ovary syndrome — the leading cause of chronic anovulation
• •Thyroid dysfunction, both overactive and underactive
• •Hypothalamic amenorrhoea from low energy availability, excessive exercise or restrictive eating
• •Raised prolactin from pituitary adenoma or medication
• •Perimenopause
• •Significant weight loss or weight gain
• •Chronic stress
• •Primary ovarian insufficiency
• •Hormonal contraception changes and post-pill adjustment
• •Uterine causes including fibroids and polyps affecting bleeding pattern
• •Chronic illness, coeliac disease and poorly controlled diabetes
• •Medications including antipsychotics, which raise prolactin

Ovulation requires a coordinated hypothalamic-pituitary-ovarian sequence, and irregular cycles almost always mean disrupted ovulation. In PCOS, elevated LH and insulin drive ovarian androgen production, arresting follicle maturation so no dominant follicle emerges. In hypothalamic amenorrhoea, low energy availability suppresses GnRH pulsatility — the body downregulates reproduction when energy is scarce. Thyroid hormones influence SHBG, prolactin and GnRH pulsatility, so dysfunction in either direction disturbs cycles. Elevated prolactin directly suppresses GnRH. Perimenopause reflects declining follicle numbers and erratic oestrogen.