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What is a menstrual cycle?

A teenager holding a pen and pointing at a menstrual calendar.

Your menstrual cycle is the time from the first day of your period to the day before your next period. The term is used to describe all the different changes your body goes through during this time.

Understanding your menstrual cycle can really help you prepare for these changes. It can also help you manage your periods better.

Trying to understand your menstrual cycle can seem really complicated, though. There are different phases of the cycle to learn and scientific words to describe your reproductive system.

But, once you start getting to know the phases and terms, it gets easier, so stick with us.

We’ve got a handy guide to some of the terms used, then a summary of why your menstrual cycle happens and how long it lasts. Lastly, we go into each phase of the menstrual cycle and what to expect during each.

Let’s break it down and explore what happens during your menstrual cycle.

Your reproductive system

Your reproductive system is made up of all the different parts of your body that allow you to get pregnant.

They include:

  • Eggs – you have a store of eggs and your body releases 1, sometimes more, each menstrual cycle.
  • Ovaries – you have 2 ovaries where your eggs are stored before they get released.
  • Fallopian tubes – each ovary has a fallopian tube next to it that take it in turns to catch an egg (or sometimes more) each cycle so that the egg can travel to your womb. If you have unprotected sex it may get fertilised on the way.
  • Womb (uterus) – this is where the egg travels to and where a baby develops if you become pregnant.
  • Cervix – this is the entrance to your womb, leading from your vagina.
  • Vagina - your vagina is the tube that goes from outside your body to your cervix.
  • Vulva – the external genitals including the labia (the lips) and clitoris.
Female reproductive system with the labels: uterus, cervix, vagina, ovary, fallopian tube

Why does the menstrual cycle happen?

Your menstrual cycle is the process your body goes through so that you can get pregnant. These changes happen every month whether or not you are planning a pregnancy or having sex.

It happens because certain hormones act on your body to release an egg. These hormones thicken the lining of your womb ready for a baby if the egg gets fertilised. If an egg is not fertilised, your womb sheds the lining it’s prepared. This comes out as your next period.

These changes in hormones during your cycle can affect the way you feel, physically and emotionally, depending on each stage. They can also affect your vaginal discharge so do not worry if that changes over your cycle. You may have more or less throughout your cycle and it can go from thick and sticky to wet and slippery.

How long does the menstrual cycle last?

Your menstrual cycle is likely to last around 28 days, but it’s also perfectly normal for it to be anywhere between 23 to 35 days, so do not worry if yours is shorter or longer. It’s more likely to be irregular in the first year or two of your periods.

The day you start your period counts as day 1 of your cycle.The last day of your cycle is the day before your next period, when a new cycle begins.

The four phases of the menstrual cycle

These phases are based on the average length of 28 days so may need adjusting for your own cycle length.

The stages of the menstrual cycle. Day 1 - 5 menstruation, day 6 - 14 Follicular phase, day 15 - 19 ovulation and day 20 - 28 luteal phase.

Menstrual phase (day 1 to around 5):

  • This phase is when you have your period if an egg is not fertilised. Your womb lining is shed and comes out of your vagina as blood and tissue. Your period may start off as bright red and turn to rusty brown by the end.
  • You may have period cramps and pain as well as bloating. You might find that pre-menstrual syndrome (PMS) symptoms like mood swings and breast tenderness ease off within a few days of your period starting. Find out more about how to manage your periods.

Follicular phase (day 1 to around day 12):

  • During your period and afterwards, hormones are already working again to prepare for the next egg, in case this one gets fertilised.
  • As soon as your period ends, hormones start to thicken the lining of your womb again.
  • You may notice you start to feel better as your period ends and may have more energy.

Ovulation (around days 13-15):

  • Your ovaries will release the next egg around halfway through your cycle, usually 14 days before your next period. This is called ovulation.
  • You are most likely to get pregnant during this phase if you have unprotected sex.
  • This phase is when your energy levels will be at their highest, and you may notice you feel at your best physically and emotionally and have more confidence. Some people may get ovulation pain.

Luteal phase (around days 16 to 28):

  • Any egg released during ovulation that’s not fertilised will still travel to your womb but hormones will start working to break down your womb lining. This lining will later be shed as your next period.
  • Energy levels may start to fall again. You may find you have pre-menstrual syndrome (PMS) symptoms like mood swings, headaches, tiredness, tender breasts, bloating and spotty skin and greasy hair.

These phases may look complicated, but thinking about how they fit in with your own cycle can help make sense of them.

And understanding what happens with each phase will give you the power to plan for your symptoms, physical and emotional. Knowing that what you’re going through is normal may help too.

Just remember that menstrual cycles can still vary from person to person. You can find out more about who to trust for information about what’s normal for periods and what’s not. You can also learn about period myths and how to manage your periods.

Watch our ambassador Dr Aziza Sesay break down the menstrual cycle