A period is the part of the menstrual cycle when a woman bleeds from her vagina for a few days. In most women this happens every 28 days or so.
It's common for women to have a cycle that occurs earlier or later than this (between 24 and 35 days).
Girls have their first period when they start puberty. This can be any age between 8 and 16, although most girls start their periods when they're about 12.
Read more about girls and puberty.
A woman's periods continue until the menopause, which usually occurs when a woman reaches her late 40s to mid-50s (the average age is 52).
The menstrual cycle
Each menstrual cycle starts on the first day of your period (day one) and lasts until the day before your next period begins.
The reproductive organs inside a woman's body consist of:
- two ovaries – where eggs are stored, developed and released
- the womb (uterus) – where a fertilised egg implants and a pregnancy develops
- fallopian tubes – the two narrow tubes that connect the ovaries to the womb
- the cervix – the lower part of the womb that connects to the vagina
- the vagina – a muscular tube leading from the cervix to outside of the body
During each menstrual cycle, levels of the hormone oestrogen rise, resulting in an egg developing and being released by the ovary (ovulation). Your womb lining thickens in preparation for a possible pregnancy.
The egg travels down the fallopian tube and if it meets a sperm and is fertilised, a pregnancy can occur.
The egg lives for about 24 hours. If it isn't fertilised, it will be absorbed into your body. The lining of your womb will come away and leave your body through the vagina mixed with blood. This is a period (it's also sometimes referred to as the menstrual flow).
Your fertile time (when you can get pregnant) can be difficult to pinpoint. It's around the time you ovulate, which for women is about 12 to 14 days before the start of the next period.
You can't get pregnant if you don't ovulate. Some hormonal methods of contraception, such as the combined pill, contraceptive patch and contraceptive injection, work by preventing ovulation.
Read more about fertility, contraception and getting pregnant.
Your body produces different amounts of hormones at different times during your menstrual cycle. This can cause changes in your body and your emotions.
For example, your vaginal secretions change throughout your menstrual cycle. Around the time of ovulation they become thinner and stretchy, a bit like raw egg white.
You may also have mood swings in the days before your period and your breasts may become swollen and painful.
Premenstrual syndrome (PMS) describes the physical, psychological and behavioural symptoms that can occur in the two weeks before your monthly period. It's also known as premenstrual tension (PMT).