What are the menstrual cycle phases, and what changes in the reproductive years?
The menstrual cycle phases (menstruation, the follicular phase, ovulation and the luteal phase) fit into a cycle of 24 to 38 days in US guidance, counted from the first day of bleeding, so the bleed itself is part of the count. The NHS in the UK describes 21 to 35 days as common, and across the reproductive years that rhythm usually settles while bone, muscle, iron needs and fertility shift quietly in the background.
If your cycle doesn't run like clockwork, that is common and usually normal, and the same goes if you are a parent wondering about your daughter's. A cycle that differs from a friend's is not a problem in itself. We find it helps to see the cycle as one of the few health signals you can read for yourself: when it changes, that is useful information, not a verdict. Below you'll find what shifts in your body during this stage, what the evidence supports, and the signs that are worth a doctor's visit.
How long does each of the menstrual cycle phases last?
A cycle is counted from the first day of one period to the first day of the next, and it moves through four phases: menstruation, the follicular phase, ovulation and the luteal phase. Because the stretch between ovulation and the next period differs from woman to woman (the table shows the range), 'ovulation on day 14' is a convenient guess rather than a rule. That variation is also why total cycle length differs so much, and why a friend's cycle and yours can both be perfectly normal.
Some women ask whether to plan training around each menstrual cycle phase, and many find it useful to treat cycle syncing as a personal experiment: note your energy and how sessions feel, then keep what works for you and drop what doesn't.
If a pregnancy is in your plans, the same phases frame your fertile days around ovulation, and our guide to a balanced pregnancy diet covers the nutrients that matter most.
Our page on signs of puberty in girls is for the years before this one. For the years after this one, when cycle length starts to shift again, see perimenopause symptoms.
| Phase | What happens | How long |
|---|---|---|
| Menstrual phase (your period) | The lining of the uterus sheds, and this starts the count. | About 5 days, with 2 to 7 days the usual range (NHS). |
| Follicular phase | An egg matures in the ovary while estrogen rises and the lining rebuilds. | The stretch before ovulation; it varies from cycle to cycle. |
| Ovulation | An ovary releases the egg, which divides the follicular phase from the luteal phase. | The point in the cycle where the egg is released, not a stretch of days. |
| Luteal phase | Progesterone rises to prepare the lining. Without a pregnancy, hormone levels fall and the next period begins. | Ovulation to the next period: about 7 to 19 days (Office on Women's Health). |
Change, evidence, what helps
Which health conditions are worth knowing about in the reproductive years?
Endometriosis, PCOS, iron deficiency and premenstrual disorders are among the conditions worth knowing about in this stage, and each has treatments that help. Our guide to women's health conditions covers them in full. Pregnancy and birth bring their own changes, and our page on postpartum exercise is there for when you feel ready to return to training.
Endometriosis and PCOS are the two that most often take time to pin down. The NHS in the UK now calls PCOS PMOS, so you may see either name. Both can show up through changes in your periods or in pelvic pain. If symptoms persist, asking your doctor (a GP in the UK) for a referral is a reasonable next step, and it is fair to ask again if the first answer doesn't feel complete.
Feeling tired in a way that rest doesn't touch has many possible explanations, and iron is one that a simple blood test can check. Our page on iron deficiency anemia treatment (spelled anaemia in the UK) explains the options, including what to do when heavy periods are the cause.
If the days before your period bring mood changes strong enough to affect work or relationships, that pattern has a name, PMDD. It also helps to know that you are not imagining it: these symptoms are recognized, and the treatments are real.
When is a cycle change worth a doctor's visit?
A cycle change is worth a doctor's visit when pain or bleeding gets in the way of your day, or when periods become unpredictable or stop. Hormones such as estrogen and progesterone shape the cycle, so a lasting change in your pattern is a reasonable thing to have checked. The NHS lists many common causes of irregular periods, and a doctor (a GP in the UK) can work out which one applies.
Menstrual cramps are common, but pain that stops you training or going about your day is worth raising rather than carrying on alone. Heavy periods are defined by their effect on your life, not by a set volume. The NHS lists changing a pad or tampon every 1 to 2 hours, needing two products at once, bleeding for more than 7 days, or passing clots of about 1 inch (2.5 cm) or larger.
Periods that stop altogether deserve attention too. A doctor may explore possibilities that include early menopause, also called premature ovarian insufficiency. In women who train hard, missing periods can be a sign of under-fueling rather than fitness, a pattern known as RED-S (IOC consensus, Mountjoy et al., 2023). Many women find that fueling well for their training, and talking it through with a doctor, helps the cycle settle into a steadier rhythm.
Facts
Common questions
How long is the menstrual cycle?
Count from the first day of bleeding to the day before the next period starts, so the days you bleed are part of the total. The US Office on Women's Health gives 24 to 38 days. In the UK, the NHS describes about every 28 days, with 21 to 35 days common. A 28-day cycle is typical, not universal.
How long is a normal period cycle?
Normal is mostly about what is steady for you, so a shift away from your usual length says more than a comparison with a friend or a 28-day textbook. Within the 24 to 38 day US range (21 to 35 days common in the UK), a cycle that runs a little longer or shorter from month to month is not unusual.
What causes irregular menstruation (periods)?
The NHS lists puberty, approaching menopause (usually ages 45 to 55), pregnancy, hormonal contraception, stress and anxiety, and excessive exercise as common causes. PCOS (called PMOS by the NHS) and thyroid problems can also be behind it. A conversation about recent life changes, plus a blood test if suggested, is a good place to start.
What's a normal cycle for a period?
Normal is a range, and its edges are where guidance suggests checking in. The Office on Women's Health advises contacting a doctor if cycles are shorter than 24 days or longer than 38, or if there is no period for 90 days when you are not pregnant or breastfeeding. The NHS suggests seeing a GP after 3 missed periods in a row with a negative pregnancy test.
Is it worth tracking your cycle?
Many women find it useful. A calendar or notes app is enough: how many days you bleed, how heavy the flow is, and any pain or mood changes alongside the day they appear. A few months of notes turns a vague worry into a clear pattern you can show a doctor, and makes any later change easier to spot.
Next steps
A good place to start is our menstrual cycle hormone chart, which shows how hormones rise and fall across the phases. From there, you might pick the one row in the table above that fits your life right now, whether that's strength training, an iron check or a conversation about pain, and take it to your doctor or OB-GYN (your GP in the UK) if anything on the red flag list sounds familiar. Your own pattern is the best guide, so this is not one size fits all: take the steps that fit your life and leave the rest.
See a doctor promptly if you have
- Pain that stops you training or going about your day
- Missing periods while training hard (a possible sign of under-fueling, not fitness)
- Pelvic pain or irregular cycles that persist: ask for a referral
Also in the reproductive years
The same topic in the stage before and after
Sources
The guidelines and studies this page draws on.
- Your menstrual cycle, US HHS Office on Women's Health (n.d.)
- Periods, NHS (n.d.)
- Irregular periods, NHS (n.d.)
- Heavy periods, NHS (n.d.)
- Pre-menstrual syndrome, NHS (n.d.)
- Morton et al.: protein and resistance training (meta-analysis), British Journal of Sports Medicine (2018)
- Mountjoy et al.: IOC consensus statement on Relative Energy Deficiency in Sport (RED-S), British Journal of Sports Medicine (2023)
- NICE NG73: Endometriosis diagnosis and management, NICE (2017)
- International PCOS Guideline, International PCOS Network (2023)
- NICE CG156: Fertility problems, NICE (2013)
- Folic acid for preventing neural tube defects, USPSTF (2023)
- ACOG Committee Opinion: physical activity in pregnancy and postpartum, ACOG (2020)
- Dietary Guidelines for Americans, 2025–2030, US Department of Agriculture and US Department of Health and Human Services (2025–2030)
- Iron, calcium, iodine and vitamin D intakes, NIH Office of Dietary Supplements (2024)