This website has been developed and fully funded by Bayer plc for a UK audience only. Disclaimer and side effect reporting information can be found at the bottom of the page.

Woman hiking in nature at one with her menstrual cycle

What is the menstrual cycle?


The menstrual cycle is defined as the time from the first day of your period to the day before your next period begins.

Calendars displaying menstrual cycle
Calendars displaying menstrual cycle


Period days

First day of period

Day before next period begins

1 full menstrual cycle


It is a recurring process and a typical cycle lasts 28 days but can occur more or less frequently (23 days to 35 days).


During this time a series of natural changes in hormone production occur along with changes to the structures that form part of the reproductive system (namely the uterus and ovaries), these changes are our body's way of preparing for the possibility of pregnancy.


If you are not pregnant, hormones send a signal to the uterus to shed its lining, this flows out of your vagina and is known as your period.

Doughnut chart showing duration of period and cycle


Click on each stage for details.

Stages or phases of the menstrual cycle
Stages or phases of the menstrual cycle
Stages or phases of the menstrual cycle
Friends discussing periods using resource found on phone


Periods start during puberty but the age from which they begin varies from person to person. What are the signs that your first period may be on the way? What else do you need to consider? Learn more, ready, set, go…


Learn more

PP-UN-WHC-GB-0042 September 2023


The health information on this site has been reviewed and approved by Bayer by an appropriately qualified medical reviewer. It is intended as general information only. It is not intended to replace a consultation with a healthcare professional, to provide specific medical advice or replace the patient information leaflet provided with your medicine. Treatments discussed here should be initiated under medical supervision. For full information including side effects and eligibility for treatment, please consult your healthcare professional. Always speak to your doctor or nurse for personal medical advice.


Images on this site are stock images and for illustrative purposes only.


Reporting side effects

If you get any side effects whilst taking a medicine, talk to your doctor, pharmacist or nurse. This includes any possible side effects not listed in the package leaflet. You can also report side effects directly via the yellow card scheme at By reporting side effects, you can help provide more information on the safety of medicines.