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Your menstrual cycle usually lasts about 28 days. But it can be as short as 20 days or as long as 36 days.

It's controlled by your hormones and it has three stages.¹

• Your cycle begins on the first day of your period. Your hormone levels are low at this time.
• In the first few days, part of your brain starts making a hormone called gonadotropin-releasing hormone (GnRH).
• GnRH tells another part of your brain to make two more hormones. They're called luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
• They travel in your bloodstream to your ovaries. Here, the hormones tell eggs to start growing.
• The growing eggs make the hormone estrogen.
• Estrogen makes the lining of your womb (endometrium) start to get thicker.
• One egg grows faster than the others. The rest shrivel up.

• In the middle of your menstrual cycle, your levels of luteinizing hormone (LH) rise sharply.
• This helps the growing egg come out of your ovary. This is called ovulation.
• The egg goes into one of your fallopian tubes. The tube's tiny hairs push the egg along, toward your womb.

• Your body gets ready for pregnancy.
• Your ovaries keep making estrogen and start to make lots of progesterone.
• Progesterone makes the lining of your womb even thicker, so it's ready for a fertilized egg.
• If a fertilized egg arrives, it may stick to your womb lining and start to grow. This is how pregnancy starts.

Two things can happen.

• You get pregnant. Your hormone levels stay high to keep your pregnancy going.
• You don't get pregnant. Your hormone levels start to drop. Without hormones, the lining of your womb breaks down, pulls away and flows out of your vagina. This is when you have your period. And your menstrual cycle starts all over 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.