A period is when blood leaves the womb through the vagina. It happens approximately every 28 days, although anywhere between 24 and 35 days is common. A period is part of a woman's menstrual cycle.
It is possible for your periods to begin between the ages of eight and 16 years, but they most commonly start around the age of 12. Your periods will continue until the menopause, which usually occurs around 45 to 55.
Puberty
As a baby girl, you are born with all your eggs stored in follicles (sacs) in your ovaries. As puberty begins, your body starts to produce hormones such as oestrogen and progesterone, which cause your eggs to mature. This is the beginning of your first menstrual cycle.
See the box, left, or watch the video for an explanation of the menstrual cycle.
Your menstrual cycle begins on the first day of bleeding, and it continues up to, but not including, the first day of your next period.
Body changes
Different amounts of each of the female hormones are produced by your body at different times during your cycle. As a result, lots of body changes occur throughout your menstrual cycle.
For example, you may develop swollen breasts and/or have mood changes. Your basal body temperature (your temperature first thing in the morning) will also change depending on which point you are at in your cycle.
