How Your Menstrual Cycle Works

A normal menstrual cycle can last from 21 to 35 days and may vary from month to month. Day one of your cycle is considered the day that your bleeding starts and it’s common for this to last up to seven days. In women who have recently started their periods, cycles can be longer and tend to get shorter with age.

Dr Simran Deo Medical Editor

Medically reviewed by

Dr Simran Deo

Last reviewed: 19 Dec 2019

The Different Phases of the Menstrual Cycle

The changes in your menstrual cycle are caused by variations in your hormones. During each cycle, your body goes through three distinct phases. The first phase of your menstrual cycle is called the menstrual period. As you move into phase two, the follicular phase, the levels of hormones called oestrogen and progestogen start to rise, and as a result an egg develops in your ovary. At the same time the lining of your uterus (womb) starts to thicken, ready for the egg to be implanted. When the egg is ready, ovulation occurs. This is where the egg is released from the ovary and travels down the fallopian tube. Ovulation marks the start of the luteal phase of your cycle.  If the egg is not fertilised by the sperm soon after release from the fallopian tube, it is absorbed back into the body, and the levels of oestrogen and progesterone drop. This drop causes the lining of the uterus to break down and be released by the body as your period. The cycle then restarts.

Which Symptoms Can Each Phase of the Cycle Cause?

The Menstrual Phase

The symptoms of the menstrual phase of your cycle start on the first day or just before you start to bleed. Most of the bleeding happens during the first three days of this phase. It may be heavy or light and you may also experience period pain. Period pain is usually described as cramping pain in the stomach. It can be a dull ache and may be quite severe. Some people find they get lower back pain and even sometimes pain which travels in the legs. Your oestrogen levels are at their lowest on day one of your period. At this time a hormone known as Follicle Stimulating Hormone (FSH) is also released from part of the brain. This signals to the ovaries that it is time to start preparing a new egg.

The Follicular Phase

This is the longest part of your cycle. The follicles – fluid-filled cavities in your ovaries – start developing. Over time, one follicle will become the more dominant and it’s in this follicle that a mature egg will grow. While this occurs, the lining of the uterus becomes thicker. Around the time of ovulation, oestrogen levels become high. This causes changes such as a white discharge in your underwear. The cloudy white mucus is from your cervix and is designed to help sperm as they are able to survive in it for 2–3 days. The last five days, leading up to and including the day of ovulation, are the most fertile days in the cycle. Extra care is needed in taking precautions, as there is a higher chance of pregnancy when having sex without contraception during this time.


Ovulation is the release of an egg from one of the ovaries. This can happen at any time from day 7 to 22 in a normal menstrual cycle, although it usually happens around day 14. You may notice a few symptoms around the time that you ovulate: red ‘spotting’ is common for a day and many women also feel discomfort in their pelvic area. This is called ‘mittelschmerz’ and is nothing to worry about. If the egg is not fertilised, the menstrual cycle restarts with a bleed. You may also notice that a day or so after you ovulate, your basal body temperature increases by a tiny amount, usually 0.1–0.2 degrees. This is because your body releases more progesterone to support the thickening of the uterus lining, which raises your temperature slightly. It usually stays elevated until the start of your next cycle, unless you become pregnant, in which case it stays higher for longer.

Luteal (premenstrual) phase

This phase follows ovulation. If conception has occurred, and the fertilised egg has implanted into the lining of your uterus, you are now pregnant. The due date is then calculated from day one of the cycle. If there is no pregnancy, the uterus lining starts to break down causing the next period. In most women (between their teenage years and the peri-menopause in their forties) the luteal phase, which takes you from ovulation to the start of the next cycle, lasts around 13 to 15 days. This premenstrual period can result in symptoms of Premenstrual Syndrome or PMS – mood changes (anger, tension, feeling emotional) and physical changes which can include:

  • water retention
  • bloating
  • painful breasts
  • acne and spots
  • headaches
  • diarrhoea
  • constipation
  • nausea
  • dizziness
  • fainting

PMS symptoms should improve when your period starts and will normally disappear a few days later. Over 100 different symptoms of PMS have been recorded, and they can change from cycle to cycle. If you’re concerned about your PMS, keep a diary of your symptoms for a few months and then speak to your doctor.

How Can I Track My Menstrual Cycle?

If you’re trying to get pregnant, or are practising natural contraception, you need to become very familiar with your own personal menstrual cycle. It helps to keep a detailed record of symptoms and signs. You can use a diary to do this or, if you prefer, use your phone to keep track of things on the go there are apps for smartphones designed for period tracking.

It’s a good idea to do this for several months so that you can see what’s normal for you.

You should keep track of:

  • The dates and length of your periods.
  • How heavy your flow is during every day of your period.
  • Any period pain or other symptoms you experience during your period.
  • Ovulation symptoms (if any) spotting/discomfort and when this happens.
  • Changes in mood or behaviour.
  • Pre-menstrual symptoms – headaches, bloating, tender breasts, acne... track anything to see if there’s a pattern.
  • Spotting prior to your period or any bleeding in-between periods

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