The menopause is when women's periods stop and they can no longer become pregnant naturally - but what else happens to the body, and why?
Menopause is a natural part of ageing which normally happens between the ages of 45 and 55, but can also be brought on by surgery to remove the ovaries or the womb (hysterectomy).
In the UK, the average age of menopause is 51.
So what's behind the change?
Hormones, specifically one called oestrogen.
It is crucial to the whole monthly reproductive cycle - the development and release of an egg from the ovaries each month for fertilisation and the thickening of the lining of the womb ready to accept the fertilised egg.
But as women age and their store of eggs declines, ovulation, periods and pregnancies stop.
And the body gradually stops producing oestrogen, which controls the whole process.
This doesn't happen overnight though.
It can take several years for this crucial hormone to fall to low levels - and then it stays that way.
What impact do hormone changes have?
The brain, skin, muscles and emotions are all affected by falling levels of oestrogen.
The body can start to behave very differently and many women experience symptoms long before their periods actually stop - during what is called the peri-menopause.
Hot flushes, night sweats, sleep problems, anxiety, low mood and loss of interest in sex are common.
Bladder problems and vaginal dryness are also normal during this time.
When oestrogen production stops altogether, there is a long-term effect on the bones and heart. Bones can weaken, making fractures more likely, and women can become more vulnerable to heart disease and stroke.
That's why women are offered hormone replacement therapy, or HRT, which boosts oestrogen levels and helps to relieve symptoms.
But not all women experience symptoms. They can also vary in their severity and how long they last - from a few months to several years.
So what causes hot flushes?
A lack of oestrogen. It is involved in the workings of the body's thermostat in the brain.
Normally, the body copes well with temperature changes, but when oestrogen is lacking, the thermostat goes wonky and the brain thinks the body is overheating when it's not.
Does oestrogen affect mood too?
Yes, it can do.
The hormone interacts with chemicals in brain receptors which control mood, and at low levels it can cause anxiety and low mood.
A lack of oestrogen can also affect the skin, making it feel dry or as if insects are crawling under the skin.
Are other hormones involved?
Yes, progesterone and testosterone - but they don't have the same impact as low levels of oestrogen.
Progesterone helps to prepare the body for pregnancy every month, and it declines when periods stop.
Testosterone, which women produce in low levels, has been linked to sex drive and energy levels.
It declines from the 30s onwards, and small numbers of women need it topped up.
So how do you know if you're going through it?
It is possible to take a blood test to measure levels of a hormone called FSH (follicle-stimulating hormones) but it's not very accurate, particularly over the age of 45.
Experts say hormone levels go up and down all the time, even during the course of a day, so the test can't really pin down what's going on.
A better way is to talk to a GP or nurse about the pattern of your periods and any symptoms you are experiencing.
Knowing what symptoms to look out for is important - feeling low and irritable needs to be recognised as much as hot flushes and night sweats.
A change in periods - becoming more heavy or more irregular - is one of the first signs of the menopause approaching.
Until you are period-free for a year, you won't know you've actually gone through the menopause.
Well, oestrogen levels in the body don't recover after the menopause.
With rising life expectancy, women are now living more than a third of their lives with oestrogen deficiency.
But there is no reason to be cowed, says Dr Heather Currie, gynaecologist, menopause expert and past chairwoman of the British Menopause Society.
"Women are continuing to work later in life, they still look amazing - the image of the menopause is changing."
Her advice: "If you're affected, go and visit your GP surgery armed with information.
"Women should know what symptoms to look out for."
She says there is plenty of support and information to help women cope with the physical and emotional changes which the menopause brings.
Hormone replacement therapy is seen as the most effective treatment available for menopausal symptoms.
There has been debate over its long-term safety, and it can cause some side-effects, but it has been shown that "the benefits of HRT outweigh the risks", Dr Currie says.
Talking to other women going through the menopause and experiencing the same symptoms is also a real help, she adds.
And the menopause is another good reason for women to lead a healthy lifestyle by:
- eating a balanced diet, low in fat and high in calcium to strengthen bones and protect the heart
- exercising regularly, to reduce anxiety, stress and guard against heart disease
- stopping smoking, to prevent heart disease and hot flushes
- not drinking too much, to reduce hot flushes
Doing these things will help to reduce the effects of the menopause on the body.