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What actually is a menstrual cycle?

It is not just your period. It is a whole monthly loop.

Cycle basics4 min read

Written by the Splash teamSources checked

On this page 5 sections
  1. Quick answer
  2. Who is in the cast?
  3. Period or cycle?
  4. How to count your cycle
  5. Sources

What actually is a menstrual cycle?

  • A cycle runs from day 1 of one period to the day before the next. The period is just the bleeding part.
  • Your brain, ovaries and womb work together through hormones.
  • Cycle length varies a lot between people. 28 days is an average, not a rule.
  • A usual period lasts 2 to 7 days and loses about 20 to 90 ml of blood.

Your period is the bit everyone talks about, but it is really just the opening scene. A menstrual cycle is the full loop your body runs, from the first day of one period to the day before the next one starts.

Who is in the cast?

  • Your brain (the hypothalamus and pituitary gland) sends hormone messages.
  • Your ovaries grow and release an egg, and make oestrogen and progesterone.
  • Your uterus (womb) builds a cosy lining, the endometrium, in case an egg is fertilised.

Every cycle, these three talk to each other in a feedback loop. Rising oestrogen thickens the womb lining and prompts an egg to be released. After that, progesterone helps the lining get ready. If no pregnancy happens, both hormones fall, the lining comes away, and that is your period. Then the loop starts again.

Period or cycle?

These two words get mixed up all the time. Your period is the bleeding. Your cycle is the whole loop, bleeding included. The NHS says a period lasts 2 to 7 days (usually about 5), is heaviest in the first 2 days, and loses about 20 to 90 ml of blood in total, which is roughly 1 to 5 tablespoons.

How to count your cycle

  1. Day 1 is the first day of real period flow. Spotting just before it is usually not counted (apps and clinics differ a little, so pick one rule and stick to it).
  2. Keep counting every day until the day before your next period starts.
  3. That total is your cycle length. Logging in Splash does the maths for you.
13%
of cycles were exactly 28 days in a study of 612,613 cycles from app users. "Textbook" is far from the norm.See sources
MythEveryone has a 28-day cycle and ovulates on day 14.
Fact

In that same study the average cycle was 29.3 days, and only about 1 in 8 cycles were exactly 28 days. Ovulation day varies from person to person and from month to month. (The study only counted cycles where ovulation could be detected, in people who chose to use an app, so treat it as a good guide rather than a final answer.)

MythYour period and your cycle are the same thing.
Fact

The period is the bleeding part. The cycle runs from day 1 of one period to the day before the next, so it is much longer.

Quick quizWhich day is "day 1" of your cycle?
  1. The day you ovulate
  2. The first day of full bleeding
  3. The last day of your period
Show the answer

B. The first day of full bleeding

Day 1 is the first day of proper period flow. Everything else in the cycle is counted from there.

Sources

  1. NHS: Periods
  2. NHS: Periods and fertility in the menstrual cycle
  3. Cleveland Clinic: Menstrual cycle
  4. Bull et al. (2019) Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles, npj Digital Medicine

Written by the Splash teamSources checked

This is general information, not medical advice. Splash can’t diagnose. If you’re worried, talk to a GP, pharmacist or sexual health clinic. In an emergency call 999 (UK) or 911 (US). How we write and check articles.

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