Menstrual cycle

The menstrual cycle is a monthly cycle where an egg is released from a woman's ovaries, and the lining of her uterus builds up and breaks down.

Stages of the menstrual cycle

 * Day 1 to Day 4: The lining of the uterus starts to break down and leave the body through the vagina on day 1. This process lasts approximately 4 days, however it can end earlier or later than this time period.
 * Day 4 to day 14: The uterus lining builds up into a thick, blood vessel filled layer. The uterus is ready to receive a fertilised egg by the end of this stage.
 * Day 14: An egg develops and is released from one of the ovaries.
 * Day 14 to day 28: The uterus lining is maintained until day 28. If the uterus lining does not receive a fertilised egg by day 28, it will break down, restarting the cycle.

Hormones and the menstrual cycle
Hormones control the menstrual cycle. Three main hormones are involved in this process; these are FSH, oestrogen, and LH. Their roles in the menstrual cycle are shown below:
 * FSH (follicle-stimulating hormone) - Causes an egg to mature in one of the ovaries and stimulates production of oestrogen from the ovaries. FSH is produced by the pituitary gland.
 * Oestrogen - Stimulates the pituitary gland to produce LH and inhibits further production of FSH. Oestrogen is produced by the ovaries.
 * LH (luteinising hormone) - Stimulates the release of an egg about halfway through the menstrual cycle. LH is produced by the pituitary gland.