The Menstrual cycle: a complete guide to all four phases

The menstrual cycle is the most under-explained system in a woman's body. Most of us learned that the period happens once a month and that we should push through. Biology tells us a different story. The menstrual cycle is a roughly four week hormonal sequence with four distinct phases, each with its own physiology, energy profile and nutrient demand. Estrogen and progesterone shift across those weeks. Pain sensitivity, sleep, mood and metabolism can shift too. Once you understand what the four phases of the menstrual cycle are and what your body is doing in each one, the rest of the picture starts to make sense. Food. Movement. Recovery. Why a hard week one feels nothing like a hard week three.

What is the menstrual cycle?

The menstrual cycle is the recurring hormonal sequence that prepares the body for a possible pregnancy. It begins on the first day of bleeding (day 1) and ends the day before the next bleed starts. Across that window the ovaries grow a follicle, release an egg, build a uterine lining and either support implantation or shed the lining. The whole sequence runs on a feedback loop between the brain (hypothalamus and pituitary) and the ovaries.^[1]

Two things often get confused. The cycle is not the period. The period is one phase of the cycle. The cycle is the full month. Day 1 is the first day of fresh bleeding. Everything else builds from there.

The cycle also has effects far beyond reproduction. Estrogen and progesterone are active in the brain, the gut, the bones, the immune system, the skin.^[24][25][8] That may be one reason why your sleep, your appetite, your stress tolerance and your training response can shift across the month.

What are the four phases of the menstrual cycle?

The cycle has four biological phases. Menstrual, follicular, ovulatory, luteal. The follicular phase technically includes ovulation, but ovulation is treated separately because the hormonal state during the fertility window is distinct.

According to a typical 28 day cycle:

1. Menstrual phase (around day 1 to 5). Estrogen and progesterone are at their lowest. The uterine lining sheds. Normal bleeding can last up to eight days.^[2] Energy is often lower. Pain sensitivity can be higher. The body is also losing iron with the blood loss.^[3]
2. Follicular phase (around day 6 to 13). FSH stimulates a cohort of follicles in the ovaries. One becomes dominant. Estrogen rises steadily, building the uterine lining. Energy, focus and stress tolerance often rise with it.
3. Ovulatory phase (around day 14 to 15). A surge of luteinising hormone (LH) triggers the dominant follicle to release its egg. The fertility window opens for roughly six days, peaking on the day of ovulation and the day before.^[4] Cervical mucus changes. Basal body temperature shifts up after ovulation.^[5]
4. Luteal phase (around day 16 to 28). The empty follicle becomes the corpus luteum and produces progesterone. Estrogen drops, then rises again to a smaller secondary peak. If pregnancy does not occur, both hormones decline as the corpus luteum regresses. That drop is what triggers menstruation. The luteal phase is when premenstrual symptoms typically show up.^[6][7]

The exact day numbers shift from woman to woman. They also shift from cycle to cycle. Real world data from over 600,000 cycles tracked through a fertility app showed an average cycle length of 29.3 days, with mean follicular length of 16.9 days and mean luteal length of 12.4 days.^[5] That is wider variation than most diagrams show. The follicular phase carries most of the variability. The luteal phase is more consistent.^[6]

Which hormones drive the cycle?

Four hormones do the heavy lifting.

• Estrogen. Rises in the follicular phase. Peaks just before ovulation. Drops, then rises again to a smaller peak in the mid-luteal phase. Estrogen builds the uterine lining, supports bone health,^[12][24] modulates serotonin and dopamine, influences skin and collagen.^[25]
• Progesterone. Low until ovulation. Rises after ovulation as the corpus luteum forms. Its metabolite allopregnanolone acts on GABA-A receptors and may contribute to calming effects in some women.^[8] Raises basal body temperature. Falls if pregnancy does not occur.^[6]
• FSH (follicle stimulating hormone). Released by the pituitary in the brain. Recruits follicles in the ovary at the start of each cycle.
• LH (luteinising hormone). Also released by the pituitary. The mid-cycle LH surge is what triggers ovulation.

These four belong to a wider hormonal context. Cortisol from the adrenal glands interacts with progesterone production. Thyroid hormones can influence ovulation and luteal function, and insulin resistance is common in PCOS (now renamed PMOS, polyendocrine metabolic ovarian syndrome).^[9][10] Vitamin D acts as a hormone in its own right. When people talk about "hormonal balance" they usually mean estrogen and progesterone. But the reality is broader and more complex than it appears on the surface.

How long should the menstrual cycle be?

The textbook number is 28 days. That is a standard set by literature, not a target. Clinical guidelines define a normal cycle as being between 24 and 38 days in adults, with bleeding lasting up to eight days.^[2] Younger women in the first few years after the start of their menstruation and women approaching perimenopause tend to have wider variation.^[11]

What matters is consistency for you. Cycles that are very short (under 24 days), very long (over 38 days), absent for more than three months in a row or vary by more than seven to nine days from cycle to cycle are worth further investigating by a clinician. They can point to thyroid issues, polyendocrine metabolic ovarian syndrome (PMOS, formerly PCOS), low energy availability, chronic stress or other conditions.^[2][12][9]

Wearables and fertility app studies have been useful to provide us more data on women's cycles. The average cycle length decreases by roughly 0.18 days per year of age between 25 and 45.^[5] None of this is something you can read from a single month. Tracking three to six months in a row gives a far more accurate picture of what is normal for your body.

Why hormones work as a system (the Hormonal System Pyramid)

One of the most useful reframes for women's health is to stop thinking about hormones as a list of separate molecules. They work together in tandem as a system. The body prioritises survival and fertility. When the foundation is unstable, the top of the system can become unregulated.

A simpler way to picture it is a four layer pyramid.

1. Layer 1: Energy and nutrients. Consistent fuel. Adequate protein. Stable blood sugar. Micronutrient sufficiency. Low energy availability can suppress ovulation.^[12]
2. Layer 2: Stress and nervous system. Cortisol and the autonomic nervous system. Sleep. Recovery. Chronic stress (emotional, physical, dietary) can suppress ovulation in some women.^[12]
3. Layer 3: Metabolic hormones. Insulin, thyroid hormones (T4 & T3), leptin. They tell the body whether energy is available and stable. In women with PMOS (formerly PCOS), insulin resistance is common and is linked to irregular ovulation.^[10]
4. Layer 4: Sex hormones. Estrogen, progesterone, FSH, LH. They sit at the top because they respond to what is happening below. Cycle symptoms can be one visible sign of strain in the lower layers.

This is why looking at one hormone in isolation can be misleading. Treating low progesterone in isolation while skipping breakfast and sleeping five hours misses the real warning signals. Your cycle is the monthly readout of the whole pyramid.

Why symptoms are signals, not flaws

Period pain is one of the most common pain conditions in women of reproductive age. A 2025 systematic review across 70 countries put the worldwide prevalence of dysmenorrhea at 71.3% (95% CI 68.7-73.8%; very low certainty of evidence).^[13] A meta-analysis estimated PMS prevalence at about 48%, but country estimates ranged from 12% to 98% depending on definitions and populations.^[14][15] Heavy menstrual bleeding is a leading cause of iron deficiency in adult women.^[3][16]

Common is not the same as biologically normal. Mild premenstrual changes and a brief drop in energy at the start of the period are within normal physiology. Cramps that stop you from working, fainting, vomiting, two week PMS that disrupts your relationships, periods that flood through pads every hour, missed periods for months at a time. These are signals worth listening to. They often respond to investigation and to addressing the underlying drivers.

Pain during menstruation is linked to inflammatory prostaglandins released as the uterine lining sheds, with secondary contributions from uterine contractions and ischaemia. Severity is influenced by family history, age at the start of menstruation, stress, sleep and nutrient status.^[17] Some of those are unchangeable. Several are not.

The point is not to chase a perfect cycle. The point is to interpret and understand what your cycle is telling you. Support your foundations. Pay attention when something shifts.

How to support each phase

Each phase has its own physiology, so some nutritional and lifestyle priorities may shift across the month. The core principles do not.

Across every phase

• Eat consistently. Three meals a day with adequate protein. Eating enough matters: low energy availability can suppress ovulation.^[12]
• Prioritise sleep. Short sleep is linked to higher dysmenorrhea risk.^[18]
• Cover the basics. Iron, magnesium, vitamin D3, folate, choline are nutrients that many women of reproductive age do not get enough of.^[26][3]

Menstrual phase

Iron is the dominant nutrient priority of the period. Normal periods shed less than 80 ml of blood (more is considered heavy bleeding), with roughly 10 to 20 mg of iron lost per cycle.^[16][19] Repeat that twelve times a year and ferritin (the iron storage marker) can drop quietly even with "normal" haemoglobin lab values.^[16] Adequate iron, vitamin C to support absorption and magnesium for muscle and nervous system support should be the priorities here.^[20]

Follicular phase

Rising estrogen may bring more energy, sharper focus and a higher threshold for stress. Some women find this a good phase for demanding work and training. Myo-inositol has been studied mainly in women with PCOS (now PMOS); evidence for effects on cycle regularity or ovulation is limited and inconclusive.^[10][21]

Ovulatory phase

The fertility window. Estrogen peaks. You may notice more energy and confidence. Real world data shows that the day of ovulation varies more than most diagrams show, so cycle tracking by basal body temperature or LH testing is more accurate than counting days.^[5]

Luteal phase

Progesterone rises. For some women this brings greater calmness and better sleep. For others, especially when the foundations are under strain, this is where PMS symptoms appear. Mood shifts, breast tenderness, sleep disruption, food cravings, water retention. Vitamin B6 has shown consistently positive effects on psychological PMS symptoms in randomised controlled trials (at doses of at least 50 mg per day), although only 1 of 31 trials had a low risk of bias.^[22] Magnesium contributes to normal muscle function and to the normal functioning of the nervous system.^[20] Vitamin D3 insufficiency is associated with specific physical premenstrual symptoms.^[23]

What this means for nōuxx Cycle Routine

nōuxx Cycle Routine is built around exactly this idea. Three formulations, one for each major hormonal state of the cycle. Bloody Berry for the menstrual phase, focused on iron and recovery. Green Glow for the follicular and ovulatory phase, focused on energy, follicle and egg quality. Calm Choco for the luteal phase, focused on magnesium and calcium. The point is not to chase every hormone. The point is to give your body the right inputs at the right time, while you keep building the foundations of health underneath.

Explore the Cycle Routine

Common questions

What is the menstrual cycle in simple terms?

The recurring hormonal sequence that prepares the body for a possible pregnancy. It runs from the first day of one period to the day before the next, with four phases driven mainly by estrogen and progesterone.

How long is a normal menstrual cycle?

For most adult women, between 24 and 38 days, with bleeding lasting up to eight days. Cycles outside this range are worth a clinical conversation. So are cycles that vary by more than seven to nine days from month to month.^[2]

What are the four phases of the menstrual cycle?

Menstrual (bleeding), follicular (rebuilding the lining), ovulatory (releasing the egg) and luteal (post-ovulation, progesterone dominant). Together they make up one full cycle.

What is the difference between the cycle and the period?

The period is the menstrual phase, the days of bleeding at the start of the cycle (up to eight days). The cycle is the entire month from day 1 of bleeding to the day before your next period starts.

Why does my cycle change in length?

Some variation is normal. The follicular phase carries most of it. Larger shifts can be triggered by stress, illness, travel, undereating, intense training, hormonal contraception coming on or off, perimenopause or thyroid changes.^[11][12][9]

Is severe period pain normal?

It is common. An estimated 71% of women report dysmenorrhea worldwide, although the studies differ widely based on the studied population.^[13] It is not biologically normal in the sense that you should accept it. Severe pain that disrupts daily life is a warning signal. Endometriosis, fibroids and adenomyosis are among the conditions that need clinical assessment.

Can nutrition really affect the cycle?

Yes. Energy availability can influence ovulation.^[12] Reviews of randomised controlled trials suggest benefit for some nutrients, such as vitamin B6, calcium and zinc, in PMS symptoms, but the quality and quantity of the evidence is unfortunately still limited (only 1 of the 31 included trials had a low risk of bias).^[22]

Should I track my cycle?

Tracking three to six months of your cycle gives you a baseline of what is normal for you. Cycle length, bleeding length, symptoms, sleep, mood and energy all carry information. Once you can identify patterns with the data you collect you're that much closer to understanding your cycle better.

References

[1] Office on Women's Health, U.S. Department of Health and Human Services. Your menstrual cycle. womenshealth.gov/menstrual-cycle/your-menstrual-cycle

[2] FIGO Menstrual Disorders Committee (Munro MG, Critchley HOD, Fraser IS). The two FIGO systems for normal and abnormal uterine bleeding symptoms and classification of causes of abnormal uterine bleeding in the reproductive years: 2018 revisions. International Journal of Gynecology & Obstetrics 2018;143(3):393-408. doi.org/10.1002/ijgo.12666

[3] Munro MG, Mast AE, Powers JM, et al. The relationship between heavy menstrual bleeding, iron deficiency, and iron deficiency anemia. American Journal of Obstetrics and Gynecology 2023;229(1):1-9. doi.org/10.1016/j.ajog.2023.01.017

[4] Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation: effects on the probability of conception, survival of the pregnancy, and sex of the baby. New England Journal of Medicine 1995;333(23):1517-1521. doi.org/10.1056/NEJM199512073332301

[5] Bull JR, Rowland SP, Scherwitzl EB, et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ Digital Medicine 2019;2:83. doi.org/10.1038/s41746-019-0152-7

[6] Reed BG, Carr BR. The normal menstrual cycle and the control of ovulation. In: Feingold KR, Adler RA, Ahmed SF, et al., eds. Endotext. MDText.com; 2018. ncbi.nlm.nih.gov/books/NBK279054

[7] Royal College of Obstetricians and Gynaecologists. Managing premenstrual syndrome (PMS) [patient information]. rcog.org.uk/for-the-public/browse-our-patient-information/managing-premenstrual-syndrome-pms

[8] Gao Q, Sun W, Wang YR, et al. Role of allopregnanolone-mediated gamma-aminobutyric acid A receptor sensitivity in the pathogenesis of premenstrual dysphoric disorder: toward precise targets for translational medicine and drug development. Frontiers in Psychiatry 2023;14:1140796. doi.org/10.3389/fpsyt.2023.1140796

[9] American Society for Reproductive Medicine, Practice Committee. Diagnosis and treatment of luteal phase deficiency: a committee opinion. Fertility and Sterility 2026;126:633-639. asrm.org/practice-guidance/practice-committee-documents/diagnosis-and-treatment-of-luteal-phase-deciency-a-committee-opinion

[10] Teede HJ, Tay CT, Laven JJE, et al. International evidence-based guideline for the assessment and management of polycystic ovary syndrome (PCOS) 2023. Monash University; 2023. doi.org/10.26180/23625288.v1

[11] O'Connor KA, Ferrell R, Brindle E, et al. Progesterone and ovulation across stages of the transition to menopause. Menopause 2009;16(6):1178-1187. doi.org/10.1097/gme.0b013e3181aa192d

[12] Gordon CM, Ackerman KE, Berga SL, et al. Functional hypothalamic amenorrhea: an Endocrine Society clinical practice guideline. Journal of Clinical Endocrinology & Metabolism 2017;102(5):1413-1439. doi.org/10.1210/jc.2017-00131

[13] de Arruda GT, Barbosa-Silva J, Driusso P, et al. Worldwide prevalence of dysmenorrhea: a systematic review and meta-analysis across 70 countries. Pain 2025;167(1):41-55. doi.org/10.1097/j.pain.0000000000003768

[14] Direkvand-Moghadam A, Sayehmiri K, Delpisheh A, Kaikhavandi S. Epidemiology of premenstrual syndrome (PMS): a systematic review and meta-analysis study. Journal of Clinical and Diagnostic Research 2014;8(2):106-109. doi.org/10.7860/JCDR/2014/8024.4021

[15] Geta TG, Woldeamanuel GG, Dassa TT. Prevalence and associated factors of premenstrual syndrome among women of the reproductive age group in Ethiopia: systematic review and meta-analysis. PLoS ONE 2020;15(11):e0241702. doi.org/10.1371/journal.pone.0241702

[16] Sholzberg M, Hillis C, Crowther M, Selby R. Diagnosis and management of iron deficiency in females. CMAJ 2025;197(24):E680-E687. doi.org/10.1503/cmaj.240570

[17] Siminiuc R, Țurcanu D. Impact of nutritional diet therapy on premenstrual syndrome. Frontiers in Nutrition 2023;10:1079417. doi.org/10.3389/fnut.2023.1079417

[18] Liu J, Wang Y, Wu L, et al. Influencing factors of primary dysmenorrhea in female college students: systematic review and meta-analysis. Medicine (Baltimore) 2024;103(49):e40906. doi.org/10.1097/MD.0000000000040906

[19] EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific opinion on dietary reference values for iron. EFSA Journal 2015;13(10):4254. doi.org/10.2903/j.efsa.2015.4254

[20] European Commission. Regulation (EU) No 432/2012 establishing a list of permitted health claims made on foods. eur-lex.europa.eu/eli/reg/2012/432/oj

[21] Fitz V, Graca S, Mahalingaiah S, et al. Inositol for polycystic ovary syndrome: a systematic review and meta-analysis to inform the 2023 update of the international evidence-based PCOS guidelines. Journal of Clinical Endocrinology and Metabolism 2024;109(6):1630-1655. doi.org/10.1210/clinem/dgad762

[22] Robinson J, Ferreira A, Iacovou M, Kellow NJ. Effect of nutritional interventions on the psychological symptoms of premenstrual syndrome in women of reproductive age: a systematic review of randomized controlled trials. Nutrition Reviews 2025;83(2):280-306. doi.org/10.1093/nutrit/nuae043

[23] Alkhalaf Z, Kim K, Kuhr DL, et al. Markers of vitamin D metabolism and premenstrual symptoms in healthy women with regular cycles. Human Reproduction 2021;36(7):1808-1820. doi.org/10.1093/humrep/deab089

[24] Toumba M, Kythreotis A, Panayiotou K, Skordis N. Estrogen receptor signaling and targets: bones, breasts and brain (review). Molecular Medicine Reports 2024;30(2):144. doi.org/10.3892/mmr.2024.13268

[25] Li Y, Deng N, Luo X, et al. Unveiling estrogen's role: a comprehensive review of its impact on skin health and disease. Apoptosis 2026;31:34. doi.org/10.1007/s10495-025-02244-4

[26] Devarshi PP, Legette LL, Grant RW, Mitmesser SH. Total estimated usual nutrient intake and nutrient status biomarkers in women of childbearing age and women of menopausal age. American Journal of Clinical Nutrition 2021;113(4):1042-1052. doi.org/10.1093/ajcn/nqaa392