Periods every three weeks: is a 21-day cycle normal?

If your periods come every 21 days or so, you are at the lower end of what the NHS describes as the usual range of 21 to 35 days. A single short cycle, or a consistent 21 to 24 day cycle that has always been your pattern, is often just how your body works. A cycle that has recently shortened, or that is now shorter than 21 days, is more likely to need explanation.

What matters most is change and accompanying symptoms. The NHS advises speaking to a GP if your periods are irregular or your normal pattern changes, if periods last longer than 7 days, if you bleed between periods, or if you have other symptoms such as weight gain, tiredness, facial hair growth or skin changes.

The cycle is counted from the first day of one period to the first day of the next. The NHS suggests using an app or diary to keep track, which shows whether cycles are consistently short or vary widely.

The NHS says a period usually lasts between 2 and 7 days and you lose about 20 to 90ml of blood in total. If a short cycle comes with a long or heavy period, the total amount of time bleeding becomes a bigger part of the month, and iron levels can be affected. The NHS lists heavy periods as needing to change a pad or tampon every 1 to 2 hours, passing clots larger than about 2.5cm, or bleeding through to clothes or bedding.

The NHS lists a number of common causes of irregular periods, and several can shorten the cycle as well as lengthen it. A short cycle does not point to one diagnosis.

For people in their 40s, shortening cycles are a very common early sign of perimenopause. The NHS says menopause usually happens between the mid-40s and mid-50s, with an average age in the UK of 51, and that periods usually become less frequent over months or years before stopping rather than ending suddenly.

The NHS menopause page says one of the first signs of perimenopause is usually a change to your periods, which may happen more often or less often, with bleeding becoming heavier or lighter. It also lists hot flushes, night sweats, sleep problems, mood changes and vaginal dryness. It advises contacting your GP if you still have periods but your bleeding pattern has changed and you are bleeding more, not less, than before.

It is possible to become pregnant during perimenopause. The NHS says contraception is still needed for 1 year after your final period if you are 50 or over, and for 2 years if you are aged 40 to 49.

Based on the NHS advice on irregular periods, heavy periods and the menopause, it is sensible to see a GP in the following situations.

• Your periods have become noticeably shorter or more frequent than they used to be, or are now irregular.

• Periods last more than 7 days, or are heavy enough to affect daily life or to make you tired or short of breath.

• You bleed between periods or after sex. The NHS says this needs to be checked by a doctor, and may be caused by infection, changes to the neck of the womb or, in rare cases, cancer.

• You have severe pain with your periods, or pain when peeing, pooing or having sex.

• You have other symptoms such as weight gain, tiredness, excess hair growth, or are struggling to get pregnant.

• You have not had a period for 12 months or more and then have any vaginal bleeding, which needs to be checked.

The NHS says treatment is not always needed for irregular periods. A GP may ask you to keep a record and may refer you to a specialist if tests are needed to find the cause. For heavy bleeding, the NHS says a series of blood tests may check for underlying conditions such as iron deficiency anaemia.

Where a cause is found, treatment follows it: the combined pill can help make periods more regular in PMOS, medicines can treat a thyroid problem, and options for heavy bleeding include an intrauterine system, tranexamic acid or anti-inflammatory painkillers such as mefenamic acid, and for some people procedures or surgery. Perimenopausal symptoms can be managed with hormone replacement therapy or other measures discussed with a GP.

If you are trying to conceive, the NHS says a short or irregular cycle can make getting pregnant more difficult because you may not ovulate regularly, and you may be referred for tests if you are struggling.