How Late Is Actually Late?
A period is considered late when it has not arrived within 5 days of when it was expected, based on your typical cycle length. Cycles vary considerably between individuals — anywhere from 21 to 35 days is within the normal range. What matters is your personal pattern. A cycle that is consistently 32 days is not irregular, even though it is longer than the textbook 28 days. A period is only “late” relative to your own baseline, not relative to someone else’s or a theoretical average.
If cycles are already irregular, the concept of “late” is more fluid. For someone whose cycles regularly vary between 28 and 40 days, a period at day 38 is not late — it is within their normal range. The concerns below apply primarily to people whose periods have become noticeably different from their usual pattern.
Stress
Psychological stress is one of the most common causes of a delayed or missed period and is the mechanism most people underestimate. The hypothalamus — the brain region that controls the hormonal cascade driving ovulation — is highly sensitive to stress signals. When cortisol (the stress hormone) is chronically elevated, it suppresses GnRH (gonadotrophin-releasing hormone), which in turn reduces FSH and LH production, and ovulation is delayed or skipped entirely. If ovulation does not occur, the period that would follow it is delayed or absent.
This means that the period you are waiting for may genuinely be late because of an exam, a work deadline, a difficult relationship situation, or a period of grief or anxiety. The delay is real and physiological, not psychosomatic. Once the stressor resolves, cycles typically normalise within 1–2 months.
Significant Weight Loss or Low Body Weight
The reproductive system is energy-sensitive. When body weight drops below a threshold that the hypothalamus interprets as insufficient energy availability for pregnancy, it suppresses the hormonal axis that drives ovulation as a protective mechanism. This is called hypothalamic amenorrhoea (HA) when periods stop entirely, or delayed ovulation when they just become irregular.
This occurs in eating disorders, extreme caloric restriction, very low body fat, and in athletes with high training loads and insufficient caloric intake — a phenomenon known as Relative Energy Deficiency in Sport (RED-S). The treatment is not medication — it is restoring adequate caloric intake and reaching a sustainable body weight. Hormonal contraception can induce withdrawal bleeds but does not treat the underlying hormonal suppression.
Excessive Exercise
Intense physical training, particularly endurance sports (long-distance running, cycling, swimming) and gymnastics, can suppress the reproductive axis independently of weight. Even women with normal BMI can develop hypothalamic amenorrhoea from very high training volumes combined with inadequate energy intake. Exercise-related menstrual dysfunction is more common than most female athletes recognise and has long-term implications for bone density (because oestrogen is needed for bone maintenance) as well as fertility.
Polycystic Ovary Syndrome (PCOS)
PCOS is the most common hormonal disorder in women of reproductive age and the most common medical reason for irregular or absent periods. In PCOS, elevated insulin and androgens disrupt follicular development — follicles begin to grow but do not reach maturity and ovulation. The result is cycles that are longer than normal (often 35–90 days), unpredictable, or absent entirely. PCOS can be diagnosed through blood tests (LH, FSH ratio, testosterone, insulin) and pelvic ultrasound. It is a lifelong condition but highly manageable with lifestyle changes and, when needed, medication.
Thyroid Disorders
Both an underactive thyroid (hypothyroidism) and an overactive thyroid (hyperthyroidism) disrupt menstrual regularity. Hypothyroidism — caused most commonly by Hashimoto’s thyroiditis, an autoimmune condition — tends to make periods heavier and irregular, and can cause amenorrhoea in severe cases. Hyperthyroidism tends to make periods lighter and less frequent. Thyroid disorders are among the most commonly missed causes of menstrual irregularity because symptoms develop gradually and can be attributed to other things. A simple TSH blood test identifies thyroid dysfunction, and treatment (levothyroxine for hypothyroidism) typically restores menstrual regularity.
Elevated Prolactin (Hyperprolactinaemia)
Prolactin is the hormone responsible for milk production. When prolactin levels are elevated outside of breastfeeding, it suppresses the hormonal axis responsible for ovulation, causing irregular or absent periods. The most common cause is a prolactinoma — a small, benign tumour on the pituitary gland that produces excess prolactin. Other causes include certain medications (antipsychotics, antidepressants, some antihistamines, blood pressure medications, and opioids). Hyperprolactinaemia is diagnosed with a blood prolactin level and, if elevated, an MRI of the pituitary. It is treatable with dopamine agonist medications (cabergoline, bromocriptine) that shrink prolactinomas and restore normal prolactin levels.
Coming Off Hormonal Contraception
When stopping hormonal contraceptives — particularly the combined pill, progestogen-only pill, or implant — periods do not always restart immediately. Post-pill amenorrhoea (absence of periods after stopping the pill) can last up to 3–6 months and is generally not concerning if it resolves within this timeframe. The pill suppresses the hypothalamic-pituitary-ovarian axis, and it takes time to reactivate after the synthetic hormones are withdrawn. However, post-pill amenorrhoea beyond 3–6 months warrants investigation because it may be revealing an underlying condition (such as PCOS or hypothalamic amenorrhoea) that the pill was masking with withdrawal bleeds.
Perimenopause
Perimenopause — the transition phase before menopause — typically begins in a woman’s mid-to-late forties but can start earlier. It is characterised by increasingly irregular cycles as ovarian function fluctuates. Cycles may become longer, shorter, heavier, lighter, or unpredictable before periods stop entirely. Menopause is confirmed after 12 consecutive months without a period. For women in their forties with changing cycle patterns, perimenopause is a common explanation. For women in their thirties experiencing similar changes, premature ovarian insufficiency (POI) should be investigated.
Premature Ovarian Insufficiency
POI (previously called premature menopause) occurs when ovarian function declines before age 40. It affects approximately 1 in 100 women. Symptoms include irregular or absent periods, hot flushes, night sweats, and reduced fertility. Diagnosis is made by finding elevated FSH levels on two separate tests at least 4 weeks apart. POI has significant long-term implications for bone density, cardiovascular health, and cognitive function because oestrogen is lost earlier than normal. Hormone replacement therapy is recommended for all women with POI until the natural age of menopause, regardless of symptoms, to protect long-term health.
When to See a Doctor
A period that is more than two weeks late in someone who has had a negative pregnancy test warrants a consultation with a doctor. Tests to consider include: pregnancy test (rule out), thyroid function, prolactin, FSH and LH, oestradiol, testosterone, AMH, and pelvic ultrasound. If cycles have always been irregular since puberty, this suggests a condition present from the outset (most commonly PCOS) rather than something that has changed. If cycles were previously regular and have become irregular, something has changed and should be identified.