A period is part of a menstrual cycle. Each month the lining of the uterus (a female reproductive organ) sheds, releasing blood through the vagina.1
Periods start at puberty and continue until menopause (the point where periods stop happening).1
Women and girls who have reached puberty have a menstrual cycle. This means that they release an egg from their ovaries roughly every 28 days.
But everyone is different, and people may find that their periods last anywhere between 23 and 35 days.
During this time, the levels of different hormones (chemical messengers) change in your body. This may affect the way you feel and the types of symptoms you experience.1
Periods last around 7 days and your menstrual cycle can be divided into four distinct phases:
The menstrual cycle begins with the menses phase. During this time, the uterus sheds its lining. This is your period. It can last anywhere between 2 and 7 days. If your periods last any longer, or if your periods are extremely heavy, contact your GP for advice.2
During the follicular phase levels of the hormones known as oestrogen and follicular-stimulating hormone (FSH) increase. Oestrogen helps to thicken the lining of your uterus. FSH makes your ovaries swell as they begin to develop fluid-filled sacs called follicles. Follicles contain mature eggs and are harmless.2
This phase of the menstrual cycle lasts around 10 days.
Luteinising hormone triggers ovulation. The surge in this hormone causes a mature egg to be released from one of your ovaries. Your oestrogen levels are also at their highest around this time.2
The luteal phase begins around day 15 of your cycle. At this stage, levels of another hormone called progesterone begin to rise. This hormone is responsible for helping to thicken your uterus lining even more.2
The mature egg begins to move along the fallopian tube. If it is fertilised by sperm, it will continue to travel through the fallopian tube and into the uterus where it will lodge in the thickened lining.2
If the egg is not fertilised by the end of the luteal phase, your oestrogen and progesterone levels will fall, and the lining of your uterus will begin to shed.2
Some people might experience physical and emotional changes during their periods. But others may not.
Premenstrual syndrome (PMS) happens when you experience changes to your mood or body before your period begins.3,4
Some of the most commonly experienced period symptoms include:
The changes that people experience may be mild or severe. If your symptoms are affecting your quality of life, talk to your pharmacist or GP. They will advise you on strategies to manage your symptoms.
Not for everyone. However, some people may experience painful cramping or migraines during their periods.
Conditions like polycystic ovary syndrome (PCOS) and endometriosis can trigger severe cramping and pain. If you live with either of these conditions or want to find out more about them, contact your GP for further information.
There are many reasons why you may experience irregular periods.
Normal changes to our bodies like puberty and menopause can trigger longer or shorter periods.
Periods stop during pregnancy. However, in the early stage of being pregnant, you may notice that you have irregular periods. If you aren’t sure whether you’re pregnant, you should take a pregnancy test to confirm.5,6
Weight loss or gain, contraception, and medical conditions may also cause your periods to become irregular.5, 6
PCOS, endometriosis, and thyroid disorders are some of the conditions that can affect your periods. Your GP can help you find out if you have any of these conditions and advise you on the treatments that may work for you.5, 6
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- Periods - NHS. Accessed March 18, 2023. https://www.nhs.uk/conditions/periods/
- Thiyagarajan DK, Basit H, Jeanmonod R. Physiology, Menstrual Cycle. In: StatPearls. StatPearls Publishing; 2022. Accessed March 18, 2023. http://www.ncbi.nlm.nih.gov/books/NBK500020/
- Premenstrual syndrome | Health topics A to Z | CKS | NICE. Accessed March 18, 2023. https://cks.nice.org.uk/topics/premenstrual-syndrome/
- Table 1, Diagnostic Criteria for Premenstrual Dysphoric Disorder (PMDD) - Endotext - NCBI Bookshelf. Accessed March 18, 2023. https://www.ncbi.nlm.nih.gov/books/NBK279045/table/premenstrual-syndrom.table1diag/
- PMS (premenstrual syndrome) - NHS. Accessed March 20, 2023. https://www.nhs.uk/conditions/pre-menstrual-syndrome/
- Irregular periods. NHS. Accessed February 27, 2023. https://www.nhs.uk/conditions/irregular-periods/
- Irregular periods | NHS inform. Accessed March 18, 2023. https://www.nhsinform.scot/healthy-living/womens-health/girls-and-young-women-puberty-to-around-25/periods-and-menstrual-health/irregular-periods
Reviewed by: Mital Thakrar
Published: April 2023
Next review: April 2026