What Is Menstrual Cycle


The menstrual cycle is a series of hormonal changes that occur in the female reproductive system (specifically the uterus and ovaries) monthly. The menstrual cycle makes way for pregnancy a possibility.1 

The cycle allows the release of egg production from the ovaries and for the preparation of the uterus for pregnancy. 

The cycle’s duration is calculated in a way: from the first day of getting the period to the first of the next period, is usually between 23 and 35 days. Menstruation typically starts taking place during puberty, estimated between the ages of 8 and 16.  Although, the average menstruation starts at around 12 years old. 

Menstruation will continue until one undergoes menopause, which takes place at around 50 years of age. An individual undergoing menopause is experiencing no menstrual period for a year. This individual is no longer fertile and hence cannot get pregnant. 

Four distinct phases are associated with the menstrual cycle: the menstrual phase, the follicular phase, the ovulation phase, and the luteal phase. 

Throughout these phases, hormone levels fluctuate into various physical and emotional changes varying from person to person.1 By understanding their menstrual cycles, individuals can gain understanding of their reproductive health, sexual health, fertility and overall well-being In addition, The ability e to recognise irregularities and potential health concerns.

What are the phases of the Menstrual Cycle?

The four phases of the menstrual cycle consist of: 2,3

  1. The menstrual phase: The menstrual cycle begins with menstruation ( period). During this period,  the body sheds the uterine lining that was built during the follicular phase. In addition, the egg released during ovulation non-fertile monthly. The average length of this phase is 3-7 days.
  2. The follicular phase:  The phase takes place at the beginning and the end of one’s period.  the phase lasts until ovulation. During this phase, the body produces oestrogen hormone, where the uterus lining thickens, and follicle-stimulating hormone (FSH). FSH stimulates the development of several follicles in the ovaries, each containing an egg. During days 10 to 14, each of these follicles has one egg developing follicles. The dominant follicle  will start to mature, becoming a fully-formed ovum 
  3. The ovulation phase occurs mid-cycle, usually around day 14 of the 28-day  cycle. A spike in production of  Luteinizing hormone (LH) causes the  ovary to release the ovum a single cell leading to ovulation
  4. The luteal phase begins after ovulation and lasts until the start of the next period. The ovum travels from the ovary through the Fallopian tubes towards the uterus. A higher level of Progesterone stimulates the uterus to receive a fertilised egg.In a scenario where is unfertilised, the corpus luteum breaks down the ovum rejecting it from the uterus alongside the unused egg and the uterine lining.  The body starts to shed to begin the cycle all over again

At what age does menstruation start?

Girls can get their first period at the age of eight or as late as 16 years old. However, on average, girls will start menstruating starting at   12 years old.9 The age factor varies across countries, races, communities and socio-economic status. With age, women will experience a reduction in hormone levels involved in the menstrual cycle and eventually experience menopause (when periods end). The onset of menopause usually occurs between the ages of 45 and 55.10

How can I track my menstrual cycle?

Tracking the menstrual cycle is advised. It is necessary for many health-related reasons. For example, managing menstrual symptoms, discovering hormone imbalances and for sexual health. Monitoring one’s cycle helps to understand the body’s natural rhythms, predicting fertile days for conception or contraception and identifying irregularities or potential health concerns. Traditionally, the calendar method can record and track the cycle:  writing down the first day and counting each day until the first day of the next period. On average, a menstruating individual loses between 20-90ml2 (1-5 tablespoons) of blood during each period.  Several products that are used by menstruating individuals to soak up menstrual bleeding:

  1. Sanitary pads
  2. Tampons
  3. Menstrual cups
  4. Period underwear

However, thanks to technology, there are a wide range of online trackers and smartphone apps that will not only track your menstrual cycle, but also track the mood, physical symptoms,  food cravings and further estimates of when the next cycle will be!

How long is a normal menstrual cycle?

On average, the menstrual cycle spans is 28 days, calculated from the first day of 

period to the first day of the subsequent period.3 However, the length of the menstrual cycle varies on womanhood individual. A regular menstrual cycle last from 21 to 35 days is also normal.4 For the first two years of menstruation, approximately 90% of cycles will typically fall within the range of  21 to 45 days.5 Occasionally, the experience of a  cycle can be shorter or longer than the range. By the third year of menstruation, around 60-80% of an individual’s cycles are more likely to align with the “normal” range observed in adults.6

What causes menstrual cycles irregularities?

An irregular menstrual cycle is a variation from the normal pattern of consistent cycle length. A menstrual cycle lasting less than 21 days is polymenorrhea, whereas those with prolonged menstrual cycles of  35 days is oligomenorrhea.11 Up to one-third of women will experience abnormal uterine bleeding in their life.12

Those that experience early ovulation have shorter cycles,7 whereas the later cycle ovulation is longer.8

Factors contributing  to irregularities in the menstrual cycle include:

  • Birth control: Some hormone-disrupting birth controls can affect the period and menstrual cycle length. Changes usually occur within the first few months;  starting a new birth control method.
  • Pregnancy: Some women may mistake implantation bleeding for their period. Implantation bleeding takes place when the fertilised egg implants into the uterine lining; a light spot bleeding occurs during the 14th days after conception
  • Ectopic Pregnancy: When a fertilised egg fails to implant into the uterus and implants into a Fallopian tube. Instead, it causes an ectopic pregnancy. The baby will not develop and serious complications happen to the mother. Ectopic pregnancies require hospital admission to prevent any severe health problems from developing in the mother
  • PCOS: Polycystic ovary syndrome (PCOS) is a condition wherein one’s periods are irregular due to hormone imbalances, causing the excess levels of male hormones or the development of fluid-filled sacs in one’s ovaries.
  • Breastfeeding: Breastfeeding a child can delay the onset of the period after giving birth, and can also cause irregularities in one’s menstrual cycle.
  • Thyroid problems: The thyroid gland produces hormones which regulate other hormones, including reproductive hormones. An underactive or overactive thyroid gland may lead to hormone imbalances and irregular period
  • Miscarriage: A spontaneous ending in pregnancy typically occurs within the first 20 weeks. In a scenario unaware of pregnancy,  miscarriages cause hormone disruptions affecting the menstrual cycle.  Stress: stress can cause hormone imbalances in the body by releasing]cortisol, a hormone-disrupting regular hormone release
  • Perimenopause: perimenopause refers to the stage before a menstruating person begins the process of menopause. Perimenopause is usually experienced in menstruating individuals in their late 30s or 40s, and involves various hormone changes and fluctuations which may cause period irregularities.
  • Diabetes: Hormones controlling the blood sugar levels insulin interacts with reproductive hormones. Diabetes, especially if left unmanaged, can lead to hormone disruptions and complications, including heavy and irregular periods

When should I see a doctor about period irregularities?

Every menstruating cycle a woman’s personal and individual hormone fluctuations throughout her menstrual cycle contribute to her own distinctive experiences; the menstrual cycle is very personalised. What may be normal to one may be seen as abnormal to someone else. Some women may experience light periods with little to no discomfort while others may experience naturally heavy periods with pain. However, see GP if you experience the following:

  • Sudden, irregular periods (women under 45 years old) are under 45 years old
  • Your menstrual cycle lasts less than every 21 days or more than every 35 days
  • Your periods last longer than seven days
  • Periods are irregular and women struggle to get pregnant. You have irregular periods managing to get pregnant
  • You have missed three missed periods in a row, getting a negative pregnancy test
  • The shortest and longest menstrual cycles differ significantly, with a gap of at least 20 days between them


The menstrual cycle is a series of hormonal changes that occur monthly in the female reproductive system, it is necessary for egg production and uterine preparation for pregnancy. It consists of four distinct phases: the menstrual phase, the follicular phase, the ovulation phase and the luteal phase. Typically, the length of the menstrual cycle length is 28 days. However, this can vary from 21 days to 35 days. Menstruation usually lasts 2 to 7 days, with an average blood loss of 30ml. The onset of menstruation can occur between the ages of 10 and 16, with an average age of 12. Tracking a menstrual cycle can provide insight into an individual’s natural hormone fluctuations and regular menstrual habits. Overall, healthy body rhythm and fertility. Irregular or heavy periods are from factors that induce uncontrolled diabetes, eating disorders, pregnancy, PCOS, thyroid issues, stress, certain medications and breastfeeding, fibroids and PID. Consult a healthcare professional if you experience sudden irregular periods, missed periods with a negative pregnancy test, fertility difficulties, excessively long periods or significant differences in cycle length. 


  1. Schmalenberger KM, Tauseef HA, Barone JC, Owens SA, Lieberman L, Jarczok MN, et al. How to study the menstrual cycle: Practical tools and recommendations. Psychoneuroendocrinology. 2021;123: 104895. https://doi.org/10.1016/j.psyneuen.2020.104895.
  2. Reed BG, Carr BR. The normal menstrual cycle and the control of ovulation. In: Feingold KR, Anawalt B, Blackman MR, Boyce A, Chrousos G, Corpas E, et al. (eds.) Endotext. South Dartmouth (MA): MDText.com, Inc.; 2000. http://www.ncbi.nlm.nih.gov/books/NBK279054/
  3. Thiyagarajan DK, Basit H, Jeanmonod R. Physiology, menstrual cycle. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2023. http://www.ncbi.nlm.nih.gov/books/NBK500020/
  4. Grieger JA, Norman RJ. Menstrual cycle length and patterns in a global cohort of women using a mobile phone app: retrospective cohort study. Journal of Medical Internet Research. 2020;22(6): e17109. https://doi.org/10.2196/17109.
  5. Fenakel K, Lurie S. The use of calcium channel blockers in obstetrics and gynecology; a review. European Journal of Obstetrics & Gynecology and Reproductive Biology. 1990;37(3): 199–203. https://doi.org/10.1016/0028-2243(90)90025-V.
  6. World Health Organization multicenter study on menstrual and ovulatory patterns in adolescent girls. II. Longitudinal study of menstrual patterns in the early postmenarcheal period, duration of bleeding episodes and menstrual cycles. World Health Organization Task Force on Adolescent Reproductive Health. Journal of Adolescent Health Care: Official Publication of the Society for Adolescent Medicine. 1986;7(4): 236–244 https://pubmed.ncbi.nlm.nih.gov/3721946/.
  7. Wilcox AJ, Dunson D, Baird DD. The timing of the “fertile window” in the menstrual cycle: day specific estimates from a prospective study. BMJ : British Medical Journal. 2000;321(7271): 1259–1262. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC27529/
  8. Mumford SL, Steiner AZ, Pollack AZ, Perkins NJ, Filiberto AC, Albert PS, et al. The utility of menstrual cycle length as an indicator of cumulative hormonal exposure. The Journal of Clinical Endocrinology & Metabolism. 2012;97(10): E1871–E1879. https://doi.org/10.1210/jc.2012-1350.
  9. Lacroix AE, Gondal H, Shumway KR, Langaker MD. Physiology, menarche. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2023. http://www.ncbi.nlm.nih.gov/books/NBK470216/
  10. What is menopause?. National Institute on Aging. https://www.nia.nih.gov/health/what-menopause
  11. Long WN. Abnormal vaginal bleeding. In: Walker HK, Hall WD, Hurst JW (eds.) Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd ed. Boston: Butterworths; 1990. http://www.ncbi.nlm.nih.gov/books/NBK282/
  12. Davis E, Sparzak PB. Abnormal uterine bleeding. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2023. http://www.ncbi.nlm.nih.gov/books/NBK532913/
  13. Sweet MG, Schmidt-Dalton TA, Weiss PM, Madsen KP. Evaluation and management of abnormal uterine bleeding in premenopausal women. American Family Physician. 2012;85(1): 35–43 https://pubmed.ncbi.nlm.nih.gov/22230306/.
  14. Heavy periods: Overview. Institute for Quality and Efficiency in Health Care (IQWiG); 2017. https://www.ncbi.nlm.nih.gov/books/NBK279294/ 
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Stephanie Adimonye

MPharm, Pharmacy, University of Brighton

Stephanie Adimonye is a clinical pharmacist with four years of experience as a GPhC registered pharmacist, specialising in community and homecare (in particular total parenteral nutrition (TPN).). Currently working in a start-up online pharmacy, she combines her clinical expertise with a business oriented mindset to ensure optimal patient outcomes. Stephanie's responsibilities include formulating individualized treatment plans, administering therapy, and monitoring patients closely. Alongside her clinical work, she is undertaking the "Writing in the Sciences" online course from Stanford University, enhancing her communication skills.

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