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 24 to 35 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 50-55."
Between the ages of 12 and 52, a woman will have around 480 periods, or fewer if she has any pregnancies.
What happens during the menstrual cycle?
To understand the menstrual cycle, it helps to know about the reproductive organs inside a woman's body. These are:
l    two ovaries (where eggs are stored, develop and are released)
l        the womb (uterus) - where a fertilised egg implants and a pregnancy grows
l        the fallopian tubes - two thin tubes that connect the ovaries to the womb
l        the cervix - the lower part of the womb that connects to the vagina
l        the vagina - a tube of muscle connecting the cervix to the outside of the body
The menstrual cycle is controlled by hormones. In each cycle, rising levels of oestrogen cause the ovary to develop an egg and release it (ovulation). The womb lining also starts to thicken.
After ovulation, the hormone progesterone helps the womb lining grow thicker, ready for pregnancy.
The egg travels down the fallopian tubes. If pregnancy doesn't occur, the egg is absorbed 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 10-16 days. Watch an animation about how the menstrual cycle works.
What are periods?
A period is made up of blood and the womb lining. The first day of a woman's period is day one of the menstrual cycle.
"Periods last around three to seven days, and women lose about three to five 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.
What happens during ovulation?
Ovulation is the release of an egg from the ovaries. A woman is born with all her eggs. Once she starts her periods, one egg (occasionally two) develops and is released during each menstrual cycle.
After ovulation, the egg lives for 24 hours. "If you release more than one egg in a month, you will produce that second egg within 24 hours of the first," says Belfield. 
Pregnancy happens if a man's sperm meet and fertilise the egg. Sperm can survive in the fallopian tubes for up to seven days after sex.
A woman can't get pregnant if ovulation doesn't occur. Some hormonal methods of contraception - such as the combined pill, the contraceptive patch and the contraceptive injection - work by stopping ovulation.
When is the fertile time?
"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 unless you're practising fertility awareness (FPA guide to natural family planning). In most women, ovulation happens around 10-16 days before the next period.
Fertility awareness can be used to plan or avoid pregnancy, but has to be taught by a trained fertility awareness instructor. It involves monitoring your vaginal secretions, taking your temperature every day, and keeping a calendar of your cycle to help pinpoint when ovulation is likely to be happening.
"It's not accurate to say that 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.
Normal vaginal secretions
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.