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Some women find their menstrual cycle isn't always regular. Their periods may be early or late, and may vary in how long they last or how heavy they are each time.
Your normal menstrual cycle can be disturbed if you change your method of contraception or if you have an imbalance of the reproductive hormones oestrogen and progesterone.
Many factors can cause a hormone imbalance, from polycystic ovary syndrome to extreme weight loss and excessive exercise.
Read more information about the causes of irregular periods.
Irregular periods are common during puberty or just before the menopause. Treatment during these times is not usually necessary.
But if you're worried because your periods are particularly heavy, last a long time or are more frequent than monthly, or if you get bleeding or spotting between periods or after sex, see your doctor.
You may need a different contraceptive or be investigated for an underlying health condition.
Read more information about treating irregular periods.
The average menstrual cycle lasts 28 days, but can vary between 24 and 35 days.
After puberty, most women develop a regular menstrual cycle, with around the same length of time between periods.
Menstrual bleeding normally lasts between two and seven days, with the average being five days.
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Your menstrual cycle can be disturbed if you change your method of contraception or you have an imbalance of the hormones oestrogen and progesterone.
It is not unusual to have a hormone imbalance for a few years after puberty and before the menopause. This can cause your menstrual cycle to become longer or shorter. Your periods may also become lighter or heavier.
If your irregular periods are caused by these age-related factors, you will not usually need to see your doctor.
The following lifestyle factors can also upset your balance of hormones and cause irregular bleeding:
An intrauterine system (IUS) or contraceptive pill may cause spotting between periods.
Small bleeds, known as breakthrough bleeds, are common when the contraceptive pill is first used. They are normally lighter and shorter than normal periods, and usually stop within the first few months.
Polycystic ovary syndrome (PCOS) occurs when very small cysts (small, fluid-filled sacs) develop in the ovaries.
The usual symptoms of PCOS are irregular or light periods, or no periods at all. This is because, in women with PCOS, ovulation (the release of an egg) may not take place as often as normal. Also, the production of hormones may be unbalanced, and you could have higher levels of testosterone than normal (this is a male hormone that women normally have a small amount of).
Read more about polycystic ovary syndrome.
Irregular bleeding can also be due to unsuspected pregnancy, early miscarriage or problems with the womb or ovaries. Your doctor may refer you to a gynaecologist (specialist in diseases of the female reproductive system) if further investigation and treatment are needed.
A thyroid disorder is another possible but rare cause of irregular periods (the thyroid gland, found in the neck, produces hormones that maintain the body's metabolism). Your doctor may test for a thyroid problem by taking a blood test to check levels of thyroid hormones in your blood.
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Irregular periods are common during puberty or just before the menopause and in these cases treatment is usually not necessary.
Talk to your doctor if you have any of the following changes in your periods:
Your doctor will ask about your periods, lifestyle and medical history to find the underlying reason for your irregular cycle.
Any necessary treatment will depend on the reason for your irregular periods.
If you have recently been fitted with an intrauterine device (IUD) and are experiencing irregular bleeding that does not settle within a few months, discuss changing to another method of contraception with your doctor or practice nurse.
If you have started taking a new contraceptive pill that is causing irregular bleeding, you may be advised to change to another type of pill.
For overweight women with polycystic ovary syndrome (PCOS), the symptoms can be improved by losing weight, which will also help with irregular periods. By losing weight, your body does not need to produce as much insulin, which reduces testosterone levels and improves your chance of ovulation (releasing an egg each month).
Other treatments for PCOS include hormone treatment and diabetes medication.
Read more information about treating polycystic ovary syndrome.
Treatment for thyroid disorders aims to return the level of thyroid hormones in your blood to normal.
You may need to take medication to stop your thyroid gland producing too many, or too few hormones.
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Your menstrual cycle should return to normal after treatment. If it does not, see your doctor.
Stress or sudden weight loss may be diagnosed as a cause of irregular periods. Relaxation techniques, stress management or counselling (talking to a therapist) may be recommended.