Why do periods occur?

Females have small organs called ovaries in the lower part of their tummy (abdomen).  The ovaries start to produce female hormones in girls around puberty which cause changes to the lining of the uterus. Every month during your period the lining of your womb is shed together with some blood.

The time between the start of one period and the start of the next is called the menstrual cycle. The average length of a menstrual cycle is 28 days, but anything between 24 and 35 days is common. During the cycle various changes occur in your body. These are caused by changing amounts of your female hormones at different times of your cycle. The menstrual cycle is split roughly into two halves:

The first half of your cycle is called the follicular or proliferative phase. The levels of your two main female hormones, oestrogen and progesterone, are low to start with and you shed the inner lining of your womb (endometrium). This causes your period (menstrual bleeding).

The second half of the cycle is called the secretory phase. After ovulation, the follicle that released the egg makes a hormone called progesterone as well as oestrogen. Progesterone causes the lining of the uterus to swell, and be ready to receive a fertilised egg. If the egg isn't fertilised, the levels of progesterone and oestrogen gradually fall. When they fall to a low level, they lose their effect on your uterus. The lining of your uterus is then shed (a period) and a new cycle then begins.