Irregular Periods: Causes and When to See a Gynaecologist

Irregular periods — cycles that arrive too early, too late, too infrequently, or not at all — are one of the most common reasons women seek a gynaecologist appointment. At Victoria Medical in London Victoria, I assess irregular menstrual cycles as a clinical signal, not a minor inconvenience. The causes range from benign hormonal fluctuations to conditions requiring prompt investigation, and distinguishing between them requires a structured clinical assessment.

Many women tolerate irregular cycles for months or years before seeking help, often because they have been told their periods are "just like that" or because they are uncertain whether the irregularity is significant. That uncertainty is understandable. But irregular periods are the body's way of signalling that something in the hormonal axis — the hypothalamus, pituitary, ovaries, thyroid, or adrenal glands — is not functioning as it should. Identifying the cause changes the clinical picture entirely.

"Irregular periods are not a diagnosis — they are a symptom. My role is to find the cause, because the cause determines the treatment, the fertility implications, and the long-term health risks."

What Is a Normal Menstrual Cycle?

A normal menstrual cycle lasts between 21 and 35 days, measured from the first day of one period to the first day of the next. Periods typically last between two and seven days, and blood loss averages 30–40 ml per cycle. Cycles outside this range — or cycles that vary by more than seven to nine days from month to month — are classified as irregular.

The clinical terms are worth knowing. Oligomenorrhoea refers to infrequent periods — fewer than nine cycles per year, or cycles longer than 35 days. Amenorrhoea is the absence of periods entirely: primary amenorrhoea means periods have never started by age 16, while secondary amenorrhoea means periods have stopped for three or more consecutive months in a woman who previously had them. Polymenorrhoea describes cycles shorter than 21 days. Each pattern points toward a different set of underlying causes.

It is also worth distinguishing irregular cycles from abnormal uterine bleeding. A woman can have regular cycles with abnormally heavy or prolonged bleeding — that is a separate clinical entity. This article focuses specifically on cycle irregularity: the timing and frequency of periods, rather than the volume or character of bleeding. If you are experiencing heavy periods alongside irregularity, the signs you should see a private gynaecologist article covers both presentations in detail.

The Main Causes of Irregular Periods

Irregular periods arise from disruption to the hypothalamic-pituitary-ovarian (HPO) axis — the hormonal signalling cascade that governs ovulation and the menstrual cycle. The disruption can originate at any point in this axis, or from external factors such as thyroid dysfunction, adrenal disease, or structural uterine pathology. The table below maps the most common causes to their clinical patterns.

Polycystic Ovary Syndrome (PCOS)

PCOS is the most common cause of irregular periods in women of reproductive age. It affects between 8% and 13% of women worldwide, though prevalence estimates vary depending on the diagnostic criteria applied3. The condition is characterised by a combination of menstrual irregularity, clinical or biochemical hyperandrogenism (excess male hormones), and polycystic ovarian morphology on ultrasound. Diagnosis requires two of these three features — the Rotterdam criteria — and no other cause of hyperandrogenism or ovulatory dysfunction2.

The menstrual pattern in PCOS is typically oligomenorrhoea — cycles longer than 35 days — or amenorrhoea. Approximately 90% of women with PCOS present with oligomenorrhoea1. The underlying mechanism is anovulation: the follicles in the ovary begin to develop but do not mature and release an egg, disrupting the normal hormonal cycle. Elevated LH relative to FSH, insulin resistance, and elevated androgens all contribute to this anovulatory state.

PCOS carries metabolic implications beyond the menstrual cycle. Women with PCOS and amenorrhoea show significantly higher rates of insulin resistance, prediabetes, and dyslipidaemia compared with those who have oligomenorrhoea or regular cycles4. This means the severity of menstrual irregularity in PCOS functions as a marker for cardiometabolic risk — a point that is often underappreciated in clinical practice. Early diagnosis and management of PCOS through a private gynaecologist consultation allows both the reproductive and metabolic dimensions of the condition to be addressed.

Associated features of PCOS include acne, hirsutism (excess facial or body hair), scalp hair thinning, and weight gain — particularly central adiposity. Not all women with PCOS have all of these features, and some women with PCOS are of normal weight. The absence of typical features does not exclude the diagnosis.

Thyroid Dysfunction

The thyroid gland plays a direct role in regulating the menstrual cycle through its influence on sex hormone-binding globulin, prolactin, and the hypothalamic-pituitary axis. Both hypothyroidism and hyperthyroidism disrupt this regulation, producing different patterns of menstrual irregularity.

Hypothyroidism — underactive thyroid — typically produces oligomenorrhoea or amenorrhoea. In a study of 233 women presenting with menstrual disorders, thyroid dysfunction was identified in 25.8% of cases, with subclinical hypothyroidism being the most common finding at 14.2%5. Subclinical hypothyroidism — defined as a raised TSH with normal free T4 — is associated with a 6.2-fold increase in the odds of menstrual disturbance compared with euthyroid women. Hyperthyroidism, by contrast, tends to produce shorter, more frequent cycles (polymenorrhoea) or lighter periods.

Thyroid hormones also influence sex steroid levels directly. Higher thyroxine concentrations are associated with greater luteal phase progesterone and follicular phase oestrogen, suggesting that even within the normal range, thyroid function affects cycle quality6. This has implications for women with subfertility and irregular cycles — thyroid function testing is a standard part of the initial investigation at Victoria Medical, included in our women's hormone blood panel.

The clinical picture of hypothyroidism extends well beyond the menstrual cycle: fatigue, weight gain, cold intolerance, constipation, dry skin, hair loss, and low mood are all common presentations. Hyperthyroidism produces the opposite pattern — weight loss, heat intolerance, palpitations, anxiety, and tremor. If you recognise these features alongside irregular periods, thyroid function testing should be the first investigation.

Functional Hypothalamic Amenorrhoea: Stress, Weight, and Exercise

Functional hypothalamic amenorrhoea (FHA) occurs when the hypothalamus reduces or stops its pulsatile release of gonadotropin-releasing hormone (GnRH) in response to energy deficit, psychological stress, or excessive physical training. The result is suppression of the entire HPO axis — LH and FSH fall, oestradiol drops, and ovulation ceases7.

The three main triggers are closely related and often coexist: low body weight or rapid weight loss, high-volume exercise (particularly endurance sports), and psychological stress. Regular aerobic exercise increases the prevalence of oligomenorrhoea from approximately 5% in sedentary women to around 20% in recreational athletes, and to 50% or more in women running 80 miles per week8. Elite athletes, dancers, and women with eating disorders are at particular risk.

FHA is not a benign condition. Chronic hypoestrogenism impairs bone mineralisation, increasing fracture risk. Cardiovascular function is also affected — endothelial dysfunction and adverse lipid profiles are documented in women with FHA. The condition is reversible, but recovery requires addressing the primary cause: reducing training load, restoring energy balance, or managing the psychological stressors driving the hypothalamic suppression7.

If your periods have become irregular or stopped in the context of significant weight loss, a new or intensified exercise programme, or a period of sustained stress, FHA is a likely diagnosis. The investigation involves measuring LH, FSH, and oestradiol — all of which will be low in FHA — alongside bone density assessment if the amenorrhoea has persisted for more than six months.

Perimenopause and the Transition to Menopause

Perimenopause — the transitional phase leading up to the menopause — typically begins in the mid-to-late 40s, though it can start earlier. The hallmark is irregular cycles: periods may become longer or shorter, heavier or lighter, more or less frequent. This variability reflects declining ovarian reserve and fluctuating oestrogen and progesterone levels as the number of remaining follicles falls.

Menopause is defined as 12 consecutive months without a period. The average age of menopause in the UK is 51. Premature ovarian insufficiency (POI) — previously called premature menopause — occurs before age 40 and affects approximately 1% of women. POI requires specific investigation and management, including hormone replacement therapy to protect bone and cardiovascular health, and fertility counselling.

The perimenopausal transition is also a time of increased endometrial risk. Anovulatory cycles produce unopposed oestrogen, which stimulates endometrial proliferation. In premenopausal women with abnormal uterine bleeding, the rate of endometrial hyperplasia or carcinoma is approximately 4.7%, rising significantly in women over 40 with intermenstrual bleeding, BMI above 25, or hypothyroidism9. Any new pattern of irregular bleeding in a woman over 40 warrants a pelvic ultrasound scan to assess endometrial thickness.

Endometriosis and Cycle Irregularity

Endometriosis is primarily associated with pelvic pain and heavy periods, but it can also cause cycle irregularity — particularly intermenstrual spotting, shortened cycles, and premenstrual spotting. The condition affects approximately 10% of women of reproductive age and carries an average diagnostic delay of four to eleven years from first symptom onset10.

The mechanism linking endometriosis to cycle irregularity is multifactorial. Endometriotic lesions on the ovaries (endometriomas) can impair follicular development and ovulation. Inflammatory mediators produced by ectopic endometrial tissue disrupt the normal hormonal environment. And the psychological burden of chronic pelvic pain — which is substantial — can itself suppress the HPO axis.

If your cycle irregularity is accompanied by significant dysmenorrhoea, deep pelvic pain, pain during intercourse, or cyclical bowel or bladder symptoms, endometriosis should be considered. A private gynaecologist consultation with pelvic ultrasound is the appropriate starting point — though it is important to know that a normal ultrasound does not exclude endometriosis, as superficial peritoneal disease is not visible on imaging.

When Should You See a Gynaecologist About Irregular Periods?

Not every variation in cycle length requires specialist assessment. Cycles can vary by a few days from month to month without clinical significance, particularly around periods of illness, travel, or stress. The question is whether the irregularity is persistent, progressive, or accompanied by other symptoms.

What Happens at a Gynaecologist Consultation for Irregular Periods?

At Victoria Medical, a consultation for irregular periods begins with a detailed clinical history. I will ask about the pattern of your cycles — when they changed, how long they have been irregular, whether there are associated symptoms — and about your medical history, family history, contraceptive use, and any recent changes in weight, exercise, or stress levels. This history is the most important part of the assessment.

Blood tests are arranged at the same appointment in most cases. The standard panel for irregular periods includes LH, FSH, oestradiol, testosterone, SHBG, prolactin, TSH, Free T4, and AMH (anti-Müllerian hormone, a marker of ovarian reserve). Depending on the clinical picture, additional tests — cortisol, DHEAS, 17-hydroxyprogesterone, or a full thyroid antibody panel — may be added.

A pelvic ultrasound scan is arranged for most women with irregular cycles. Transvaginal ultrasound provides the most detailed view of the uterus and ovaries, assessing for polycystic ovarian morphology, endometrial thickness, fibroids, polyps, and ovarian cysts. At Victoria Medical, ultrasound can be performed on the same day as your consultation.

Results are reviewed at a follow-up consultation, at which point a diagnosis is made and a management plan is discussed. Treatment depends entirely on the underlying cause — there is no single treatment for irregular periods, because the cause determines the approach.

Treatment Options: Matched to the Cause

Irregular Periods and Fertility

Irregular cycles are one of the most common presentations in women who are trying to conceive without success. The link is straightforward: irregular cycles usually reflect irregular or absent ovulation, and without ovulation, conception is not possible through natural means. Identifying and treating the cause of the irregularity is therefore the first step in any fertility assessment.

PCOS accounts for approximately 70–80% of cases of anovulatory infertility. Thyroid dysfunction, hyperprolactinaemia, and FHA together account for a significant proportion of the remainder. In most cases, treating the underlying condition restores ovulation and allows natural conception. Where it does not, ovulation induction or assisted reproduction may be appropriate — but these interventions are most effective when the underlying cause has been identified and managed.

AMH (anti-Müllerian hormone) measurement, included in our women's hormone blood panel, provides an estimate of ovarian reserve — the number of remaining eggs. This is particularly relevant for women with irregular cycles who are planning to conceive, as it informs the urgency of investigation and the likely response to fertility treatment.

Frequently Asked Questions

References

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