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Your menstrual cycle lasts about 28 days but it can be shorter or longer. It's controlled by your hormones. The cycle has three stages.¹

• Your cycle begins on the first day of your period. At this time, you have low levels of hormones.
• In the first few days, part of your brain starts making a hormone called gonadotropin-releasing hormone (or GnRH for short).
• GnRH tells another part of the brain to produce two more hormones. They're called luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
• LH and FSH travel in your bloodstream to your ovaries. Here, the hormones tell some eggs to start growing.
• The growing eggs make two more hormones called estrogen and progesterone.
• One egg grows faster than the others. This egg keeps growing and the others shrivel up. This tends to happen in alternate ovaries each month.

• In the middle of your menstrual cycle, there's a big increase in the amount of luteinizing hormone (LH) in your body.
• This helps the growing egg move out of your ovary into your fallopian tube. This is called ovulation.
• Tiny hairs in the tube push the egg along the tube, toward your womb.

• At the end of your cycle, your body prepares for pregnancy.
• Your ovary starts to make large amounts of a hormone called progesterone.
• This hormone makes the lining of your womb thicker, ready for a fertilized egg to arrive. If an egg arrives, it joins to the lining of your womb. This lining is called the endometrium.

Two things can happen at the end of your cycle:

• You get pregnant. The levels of hormones in your body stay high to continue your pregnancy.
• You don't get pregnant. Hormone levels start to drop. Without hormones, the lining of your womb begins to break down and fall toward your vagina. This is when you have your period. When your period has finished, hormone levels start to rise and the cycle starts again.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.