The length of the menstrual cycle varies from woman to woman, but the average is to have periods every 28 days. Regular cycles that are longer or shorter than this, from 21 to 40 days, are normal.
"The menstrual cycle is the time from the first day of a woman's period to the day before her next period," says Toni Belfield, a specialist in sexual health information, and a trained fertility awareness teacher.
"Girls can start their periods anywhere from age 10 upwards, but the average is around 12 years," says Belfield. "The average age for the menopause (when periods stop) in this country is 50 to 55."
Between the ages of 12 and 52, a woman will have around 480 periods, or fewer if she has any pregnancies.
Read more about starting periods.
To understand the menstrual cycle, it helps to know about the reproductive organs inside a woman's body. These are:
The menstrual cycle is controlled by hormones. In each cycle, rising levels of the hormone oestrogen cause the ovary to develop and release an egg (ovulation). The womb lining also starts to thicken.
In the second half of the cycle, the hormone progesterone helps the womb to prepare for implantation of a developing embryo.
The egg travels down the fallopian tubes. If pregnancy doesn't occur, the egg is reabsorbed into the body. Levels of oestrogen and progesterone fall, and the womb lining comes away and leaves the body as a period (the menstrual flow).
The time from the release of an egg to the start of a period is around 10 to 16 days. Watch an animation about how the menstrual cycle works.
A period is made up of blood and the womb lining. The first day of a woman's period is day 1 of the menstrual cycle.
"Periods last around 2 to 7 days, and women lose about 3 to 5 tablespoons of blood in a period," says Belfield.
Some women bleed more heavily than this, but help is available if heavy periods are a problem.
Find out about treatments for heavy periods.
Ovulation is the release of an egg from the ovaries. A woman is born with all her eggs.
Once she starts her periods, 1 egg develops and is released during each menstrual cycle. After ovulation, the egg lives for 24 hours.
Pregnancy happens if a man's sperm meet and fertilise the egg. Sperm can survive in the fallopian tubes for up to 7 days after sex.
Occasionally, more than 1 egg is released during ovulation. If more than 1 egg is fertilised it can lead to a multiple pregnancy, such as twins.
A woman can't get pregnant if ovulation doesn't occur. Some methods of hormonal contraception – such as the combined pill, the contraceptive patch and the contraceptive injection – work by stopping ovulation.
"Theoretically, there's only a short time when women can get pregnant, and that is the time around ovulation," says Belfield.
It's difficult to pinpoint exactly when ovulation happens but in most women, it happens around 10 to 16 days before the next period.
"It's not accurate to say that all women are fertile on day 14 of the menstrual cycle," says Belfield. This might be true for women who have a regular, 28-day cycle, but it won't apply to women whose cycles are shorter or longer.
For more information on fertility awareness, see the FPA guide to natural family planning.
Vaginal secretions (sometimes called vaginal discharge) change during the menstrual cycle. Around the time of ovulation, they become thinner and stretchy, a bit like raw egg white.
See your GP if you are concerned about a change in your vaginal discharge.