Irregular periods: causes, red flags and when to seek help

The NHS gives the average gap between periods starting as around 28 days, and says it can be a bit shorter or longer. A woman has irregular periods if the gap is less than 21 days or more than 35 days. Day 1 of the cycle is the first day of bleeding, and the cycle ends the day before the next period starts.

Irregular does not only mean late. It includes cycles of widely varying length from one month to the next, bleeding that is much heavier or lighter than usual, periods that stop for months, and unpredictable spotting between periods. A tracking app or paper diary for three or four cycles turns a vague worry into a pattern that a clinician can interpret.

The NHS lists puberty, the start of menopause (usually between the ages of 45 and 55), pregnancy (a missed period is often an early sign), hormonal contraception such as the progestogen-only pill, contraceptive injection and intrauterine system, losing or gaining a lot of weight, stress and anxiety, and exercising too much as common causes. It adds that conditions like PMOS, which used to be called polycystic ovary syndrome (PCOS), or thyroid problems can sometimes cause irregular periods.

The NHS describes polyendocrine metabolic ovarian syndrome (PMOS) as a hormone condition that can affect hair growth, periods, fertility and mood, with irregular periods or long gaps between periods among its main symptoms, together with excessive hair growth or hair loss, weight gain or difficulty losing weight, difficulty getting pregnant, depression or anxiety, oily skin and acne, tiredness and thick dark patches on the neck or armpits. It says tests include blood tests for hormone levels and insulin resistance and, for over-18s, an ultrasound of the ovaries.

An overactive thyroid, which the NHS says can cause an irregular or unusually fast heart rate, weight loss, nervousness, sensitivity to heat and trembling, and an underactive thyroid, which causes tiredness, weight gain and feeling cold, can both disturb the cycle. Other possibilities include premature ovarian insufficiency, raised prolactin, and structural problems in the womb.

The NHS says to see a GP if your periods are irregular, last longer than 7 days, come with other symptoms like weight gain, tiredness, hair growth on your face and dry or oily skin, if you have bleeding between periods, or if your periods are irregular and you are struggling to get pregnant. From the menopause pages, it advises contacting a GP if you still have periods but your bleeding pattern has changed and you are bleeding more, not less, than before, or if you have not had a period for 12 months or more and have any vaginal bleeding.

Seek urgent help for very heavy bleeding that soaks through protection every hour, with dizziness or faintness, severe pain on one side of the lower abdomen with a late period, which can signal ectopic pregnancy, or fever with pelvic pain and unusual discharge. A positive pregnancy test with bleeding and one-sided pain needs same-day assessment.

Irregular bleeding after sex, any bleeding after menopause, or persistent bleeding between periods in someone over 45 deserves prompt evaluation, since uterine and cervical causes must be excluded.

Be prepared to describe your cycle length, first day of your last period, flow, pain, contraception, sexual activity, weight changes, exercise, stress, medicines, family history of thyroid disease or early menopause, and any hair, skin or milk-production changes. A pregnancy test is usually the first step if you could be pregnant.

The clinician may check weight and blood pressure, look for signs of excess hair, acne, skin darkening and thyroid enlargement, and perform an abdominal and, if indicated, a pelvic examination or cervical screening.

Tests depend on the clues: a pregnancy test, blood tests for thyroid function, prolactin, FSH and LH, testosterone and sometimes glucose, and a pelvic ultrasound. The NHS says treatment is not always needed for irregular periods, that the GP may refer you to a specialist if tests are required, and that treatment depends on the cause. For PMOS the combined pill may help make periods more regular, and if another hormone problem such as a thyroid problem is responsible, medicines can treat it.

The NHS adds that getting pregnant can be more difficult with irregular periods because you may not ovulate regularly, that having sex every 2 or 3 days throughout the cycle can help, and that if you are struggling you may be referred for further tests and offered hormone medicine or fertility treatment.