On the blob
When a girls starts her periods it's often hailed as the first step towards becoming a woman. Monthly bleeding from the vagina first begins during teenage years and continues until a woman is in her forties or fifties.
Periods are caused by changing hormone levels that fluctuate in a regular cycle that controls fertility. This cycle is known as the 'menstrual cycle'. The average length of the menstrual cycle is 28 days, but ranges anywhere between 19 and 37 days.
The menstrual cycle
Day one of the menstrual cycle is marked by the beginning of a period, also known as 'menstruation'. During menstruation, the thick uterus (womb) lining, which built up during the previous menstrual cycle, is released along with blood and mucus. The uterus contracts to force out the old lining.
This can also cause stomach cramps in some women. On average only 40ml of blood are lost during a period, but the amount can be five times more or five times less. Periods last around 5 days.
After menstruation, rising levels of sex hormones trigger an egg to mature in one of the ovaries. Meanwhile, the lining of the uterus prepares to receive an embryo. It becomes at least five times thicker and is filled with blood and mucus.
Half way through the menstrual cycle, a mature egg is released from one of the ovaries. If this egg is fertilised, pregnancy will follow. Fertilisation will only occur if there has been unprotected sex. In most cases, the egg is not fertilised and the menstrual cycle will begin again.
In many girls, the start of bleeding does not mean that sexual maturity has been reached. Often there will be no egg released for the first month. On the other hand, some girls will release eggs for several months before bleeding occurs.
Pre-Menstrual Syndrome, or PMS, is the name for some of the symptoms girls experience in the days leading up to a period. These include breast pain and water retention. A girl might also become irritable and tired, or suffer from nausea and headaches.
More pleasant perhaps is a craving for chocolate and stodgy food. As many as three quarters of women experience some form of PMS and it can be worse during teenage years as the menstrual cycle is becoming established.