What a normal menstrual cycle looks like, and why it counts as a vital sign

Key takeaways

• In 2015, the American College of Obstetricians and Gynecologists urged clinicians to treat the menstrual cycle of girls and teens as a vital sign, a regular reading that can flag health problems early¹.
• In a company-funded analysis of 612,613 app-logged cycles, only 13% lasted exactly 28 days, and ovulation often came later than day 14⁴.
• Federal health agencies put normal adult cycles at roughly 21 to 38 days, and teens’ cycles can run 21 to 45 days for a few years after the first period²³.
• In July 2026, the FDA said its own testing found trace metals in tampons but concluded that too little escapes during use to cause harm⁸¹¹.
• Toxic shock syndrome is rare, but a sudden high fever, vomiting or a sunburn-like rash during a period means remove the tampon or cup and get emergency care²⁸⁹.

In December 2015, the American College of Obstetricians and Gynecologists (ACOG) asked doctors to think about a girl’s period the way they think about her pulse or blood pressure¹. The group called the menstrual cycle a vital sign: a reading that can reveal health problems early, some of which carry into adult life¹.

ACOG also urged clinicians to teach girls and their families what a normal cycle looks like¹. Many adults may have missed that lesson too. The opinion was written about girls and teens, but its core idea, that a cycle can flag problems early, is why this guide borrows the phrase for readers of every age.

The version most of us learned, a 28-day cycle with ovulation on day 14, is an average, not a rule. When researchers at a fertility-app company analyzed 612,613 cycles logged by about 125,000 users, only 13% lasted exactly 28 days⁴.

That matters if you value knowing your own body. Your cycle is data you collect for free, every month, without a lab. Reading it well means knowing the real ranges, what shifts them and which signals deserve a call to a clinician.

This guide covers how a cycle works, what counts as normal at different ages, what large app studies added, and the facts behind three product worries: toxic shock, metals in tampons and chemicals in period underwear.

A cycle has two halves, and the first half does most of the stretching

A cycle starts on the first day of bleeding and ends the day before the next period starts². In the classic 28-day example from the National Institute of Child Health and Human Development (NICHD), bleeding fills roughly days 1 to 5³.

The follicular phase. After bleeding ends, estrogen climbs, the lining of the uterus thickens and egg-holding sacs called follicles mature in the ovaries³. Estrogen is one of the two main hormones that rise and fall across the cycle².

Ovulation. A surge of luteinizing hormone (LH) triggers the release of an egg. The HHS Office on Women’s Health (OWH) says LH surges about 36 hours before ovulation². NICHD notes that ovulation can happen anywhere from about day 10 to day 21³. Urine LH tests and a slight rise in body temperature can help track it³.

The luteal phase. After ovulation, progesterone, the second main hormone, rises and helps ready the lining for a possible pregnancy. If no pregnancy occurs, hormone levels fall and the lining sheds as your period²³. OWH puts the time from ovulation to the next period at 7 to 19 days².

The two halves don’t stretch equally. In the app analysis of 612,613 cycles, the follicular phase averaged 16.9 days and varied widely, while the luteal phase averaged 12.4 days and stayed steadier⁴. When a cycle runs long or short, the first half usually explains it.

With the follicular phase averaging nearly 17 days, ovulation typically fell after day 14, and the authors called the day-14 belief incorrect⁴. Natural Cycles, the company behind a birth control app, funded the study, and most of its authors worked for or founded the company⁴. Later articles on estrogen and progesterone explain what each hormone does.

Normal is a range, and the range changes with age

Federal agencies don’t draw the lines in quite the same place. NICHD says most women’s cycles run 21 to 35 days³. OWH calls any cycle from 24 to 38 days regular². Both agree that teens’ cycles run longer and less predictably for a few years after the first period, and NICHD puts the teen range at 21 to 45 days²³.

Use the chart below as a guide. Your own pattern over several months tells you more than any single cycle.

Is my cycle in the normal range?

This chart is our own, built from the sources cited in each row.

What to track	Typical range	Ask a clinician if	Source
First period	About age 12 on average; ages 8 to 15	It starts before 8, hasn’t come by 15, or hasn’t come within 3 years of breast growth	2
Cycle length, first few years	21 to 45 days	Cycles are still irregular more than 3 years after the first period	23
Cycle length, adults	21 to 35 days (NICHD) or 24 to 38 days (OWH)	Cycles keep coming sooner than 24 days or later than 38	23
Change from cycle to cycle	A few days	Lengths swing by more than 20 days	34
Days of bleeding	About 5 on average	Bleeding often lasts more than 8 days	23
Amount of blood	About 2 to 3 tablespoons per period	You soak a pad or tampon every 1 to 2 hours, or pass clots bigger than a quarter	2
Missed periods	None, unless pregnant, breastfeeding or using some hormonal methods	No period for 90 days when you aren’t pregnant or breastfeeding	23
Pain	Cramps that ease with simple care	Pain or bleeding keeps you from school, work or normal activities	2

Menstrual irregularities are common. NICHD estimates that 14% to 25% of women of childbearing age have them³. If heavy bleeding fits your pattern, our iron article explains how it can drain your stores.

What 165,000 app-logged cycles added

Phones have turned period tracking into giant research datasets. One U.S. example is the Apple Women’s Health Study, run by researchers at the Harvard T.H. Chan School of Public Health⁵⁶.

In 2023, the team reported on 165,668 cycles from 12,608 U.S. participants⁵. They left out people who reported polycystic ovary syndrome, fibroids, a hysterectomy or current hormone use⁶. The average cycle was 28.7 days⁶.

Age shaped the pattern. Average length was 30.3 days for people under 20, 28.7 days at 35 to 39, and 30.8 days over 50⁶. Cycles were most predictable at 35 to 39, when a person’s cycle length varied by about 3.8 days on average. That swing was 5.3 days under 20 and 11.2 days after 50⁶. The cycle is least predictable at its two ends, early in life and near menopause.

So did background and body size. Compared with white participants, Asian participants’ cycles ran 1.6 days longer and Hispanic participants’ ran 0.7 days longer⁵. People with a body mass index of 40 or more had cycles 1.5 days longer than those in the 18.5 to 25 range⁵.

The differences are small, but the authors note that today’s normal ranges were built mostly from studies of white women⁶⁷.

Participants were English-speaking iPhone users who logged their own periods, with no hormone tests to confirm ovulation⁷. The paper says its funder supplied the platform and tools for collecting data and took part in writing the manuscript, though not in the analysis; the data sat on Apple-hosted servers⁷.

The fertility-app analysis added a different finding. Between ages 25 and 45, cycles shortened by about 0.18 days a year, or about 3.6 days over those 20 years by our math, because the follicular phase got shorter⁴. The luteal phase barely changed with age⁴. That analysis also excluded 48% of logged cycles because no ovulation was detected, which could skew the results⁴.

If you use an app, your own history is its most useful output. Who else sees that history is a separate question, covered in our app-privacy guide and a later article on cycle-tracking apps.

When a missing period is a message

A steady pattern is reassuring, and a change is worth noticing.

NICHD lists the conditions behind irregular or light periods. They include eating disorders, excessive exercise, thyroid problems, high levels of the hormone prolactin, uncontrolled diabetes and approaching menopause³. Hormonal birth control and some medicines can change periods, too³.

OWH says to see a doctor if you go 90 days without a period when you aren’t pregnant or breastfeeding². Don’t write it off as a side effect of training hard or eating light. Our article on when exercise does harm explains how too little fuel can disrupt hormones and bones.

If missed periods are tied to eating too little or exercising compulsively, the National Alliance for Eating Disorders helpline, 1-866-662-1235, offers support and referrals on weekdays.

Heavy or painful periods are signals, too. Our companion article on period symptoms rates the options for cramps, PMS and heavy flow, and a later article covers which reproductive symptoms need prompt assessment.

Toxic shock is rare, but its warning signs call for speed

Toxic shock syndrome (TSS) is an illness caused by a toxin some bacteria make. The toxin can damage organs, send the body into shock and kill⁹. The FDA links it to tampon use, especially when a tampon stays in too long⁸.

The FDA says reported cases have dropped sharply over the past 20 years, partly because of labeling and education⁸.

Know the signs. The FDA lists a sudden fever, usually 102°F or higher, vomiting, diarrhea, fainting or near-fainting when you stand, dizziness and a rash that looks like sunburn⁹. These matter most during or just after a period⁹.

Act fast. OWH says to call 911 or go to a hospital for TSS symptoms, removing any tampon, cup, sponge or diaphragm first². The FDA says to take the tampon out and contact your provider or go to the nearest emergency room⁸.

Lower the risk. The FDA’s advice⁸⁹:

• Use the lowest absorbency you need. If one tampon lasts 8 hours, it may be more absorbent than you need.
• Change tampons every 4 to 8 hours, even on light days.
• Don’t leave any single tampon in for more than 8 hours.
• Use tampons only during your period, not for any other purpose.

Cups, discs and reusables. Menstrual cups and discs are regulated by the FDA as medical devices, like tampons and pads¹⁰. OWH’s toxic shock warning names cups too². The FDA hasn’t authorized any reusable tampons and discourages their use⁸.

You can report a problem with a tampon or other period product to the FDA’s MedWatch program⁹¹⁰.

Metals in tampons: what the 2024 study found and what the FDA added

In July 2024, a team led by Jenni Shearston of the University of California, Berkeley, with Columbia and Michigan State researchers, published tests of 30 tampons from 14 brands in the journal Environment International¹². The tampons were bought in New York City, London and Athens¹².

The researchers found measurable amounts of all 16 metals they looked for, including lead, arsenic and cadmium¹². Lead ran higher in non-organic tampons and arsenic higher in organic ones. No category, organic or not, store brand or name brand, was consistently lower¹².

The study had a key gap: it didn’t test whether any metal leaves a tampon and enters the body¹². At the time, the FDA only recommended that tampons be free of pesticide residue and dioxin, NPR reported¹².

The FDA then commissioned an outside review of published research in 2024, covering nine studies, and ran its own lab tests¹⁰¹¹. Agency scientists tested 11 legally sold tampon products under deliberately harsh conditions, holding them in acidified water at 50°C (122°F) for 24 hours¹¹. They found trace amounts of lead, arsenic and cadmium among other metals¹⁰¹¹.

The risk assessment, published in the journal Toxicological Sciences and announced in July 2026, found margins of safety of 7 or greater¹¹. In plain terms, the estimated exposure stayed at least seven times below the level of concern, even under exaggerated conditions. The FDA concluded that tampons release too little metal during use to cause harm⁸.

In October 2025, the FDA issued draft guidance on testing menstrual products for contaminants and listing ingredients on labels. It will replace 2005 guidance once finalized, and the agency’s July 2026 pages still listed it as a draft⁸¹⁰.

For natural-health readers, one lesson stands out. In the 2024 study, an organic label didn’t guarantee lower metals across the board¹².

PFAS in period underwear: a settlement, not a safety verdict

PFAS are human-made chemicals that break down very slowly. In January 2023, the period-underwear company Thinx settled a class action claiming it used them to boost performance features such as moisture-wicking¹³.

The suit alleged deceptive marketing, not physical harm. Thinx agreed to pay up to $5 million, at $7 per pair for up to three pairs, which comes to $21 at most by our math¹³. The company denied wrongdoing and agreed to make sure PFAS aren’t intentionally added at any stage of production¹³.

The CDC’s toxic substances agency says high levels of certain PFAS may be linked to higher cholesterol, liver changes, lower birth weight and kidney or testicular cancer, NPR reported¹³. We found no study showing harm from wearing period underwear. The FDA counts period underwear with pads under its medical device rules¹⁰.

Cycle syncing: a tidy idea the trials haven’t backed

Cycle syncing means timing workouts, meals or work to your cycle’s phases: hard training in the first half, gentler sessions later, and so on.

In 2023, researchers at McMaster University and Manchester Metropolitan University reviewed the published reviews on cycle phase and strength¹⁴. They concluded it was too early to say that monthly hormone swings meaningfully change strength or muscle gains¹⁴. They called phase-based training plans not evidence-based and blamed inconsistent findings on weak studies¹⁴.

They also described a 2020 analysis of 73 studies with 954 participants. It found that performance might dip during the early follicular phase, around your period, but only by a trivial amount¹⁴.

None of the reviews we found tested cycle-timed eating plans. What the researchers did suggest is simple: fit training to the person, and keep notes on symptoms to guide changes over time¹⁴. That’s cycle awareness without a rulebook.

What the evidence says

Claim	What the evidence shows	Rating
Normal adult cycles vary in length	Federal ranges span about 21 to 38 days; app studies agree235	Strong evidence
Ovulation reliably falls on day 14	Tested and not supported: only 13% of 612,613 app cycles were 28 days, and ovulation often came later34	Evidence says no, or unsafe
Cycle length shifts with age	Two large app datasets agree45	Strong evidence
TSS is tied to tampons left in too long	FDA’s assessment; reported cases fell after labeling and education8	Strong evidence
Tampons release harmful amounts of metal	Tested and not supported: FDA testing under harsh conditions found released amounts too small to cause harm811	Evidence says no, or unsafe
PFAS in period underwear harm health	Settlement over marketing; no health study found13	Not enough evidence
Cycle-synced training boosts results	Reviews find no clear effect of phase14	Not enough evidence

Questions readers ask

The bottom line

Your cycle is one of the few health readings you can take yourself, every month, for free. Normal is a range rather than a single number, and it shifts with age. Your own pattern over several months is the best reference you have.

Most period-product fears deserve calm, not panic: the FDA’s testing found tampon metals too low to cause harm. Toxic shock is the exception that calls for speed, so know its signs. And a missed or changing period is information worth acting on, not ignoring.

Related reading

• Heat and pain relievers lead for cramps, while PMDD is a different problem
• More People Now Qualify as Low in Iron. Find the Cause Before the Pill
• Your wearable’s best number is your own normal, not the average
• Health apps share more than you think, and you can lock yours down
• When exercise does harm, and why a famous too-much-jogging study rested on two deaths

Sources

1. American College of Obstetricians and Gynecologists. “ACOG Committee Opinion No. 651: Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign.” Obstetrics & Gynecology, 2015;126(6):e143–e146 (PubMed abstract). pubmed.ncbi.nlm.nih.gov
2. HHS Office on Women’s Health. “Your menstrual cycle.” Updated January 13, 2025. womenshealth.gov
3. Eunice Kennedy Shriver National Institute of Child Health and Human Development. “Menstruation and Menstrual Problems.” NICHD fact sheet. nichd.nih.gov
4. Bull JR, Rowland SP, Berglund Scherwitzl E, Scherwitzl R, Gemzell Danielsson K, Harper J. “Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles.” npj Digital Medicine, 2019 (University College London repository copy). pubmed.ncbi.nlm.nih.gov
5. Apple Women’s Health Study investigators. “Menstrual cycle length variation by demographic characteristics from the Apple Women’s Health Study.” npj Digital Medicine, 2023;6:100 (PubMed abstract). pubmed.ncbi.nlm.nih.gov
6. Harvard T.H. Chan School of Public Health. “Menstrual Cycles Today: How Menstrual Cycles Vary by Age, Weight, Race, and Ethnicity.” Apple Women’s Health Study update. hsph.harvard.edu
7. Apple Women’s Health Study investigators. “Variation in menstrual cycle length by age, race/ethnicity, and body mass index in a large digital cohort of women in the US.” medRxiv preprint, 2022. doi.org
8. U.S. Food and Drug Administration. “Tampons.” Content current as of July 6, 2026. fda.gov
9. U.S. Food and Drug Administration. “The Facts on Tampons—and How to Use Them Safely.” Consumer Update, content current as of September 15, 2026. fda.gov
10. U.S. Food and Drug Administration. “Menstrual Product Options, Facts, and Safe Use.” Content current as of July 6, 2026. fda.gov
11. Al-Faruque F. “Tampons: FDA toxicological study finds negligible levels of toxic metals.” Regulatory Focus, Regulatory Affairs Professionals Society, July 9, 2026. raps.org
12. Treisman R. “A study found toxic metals in popular tampon brands. Here’s what experts advise.” NPR, July 11, 2024 (via Connecticut Public). ctpublic.org
13. Treisman R. “Thinx settled a lawsuit over chemicals in its period underwear. Here’s what to know.” NPR, January 19, 2023 (via WUNC). wunc.org
14. Colenso-Semple LM, D’Souza AC, Elliott-Sale KJ, Phillips SM. “Current evidence shows no influence of women’s menstrual cycle phase on acute strength performance or adaptations to resistance exercise training.” Frontiers in Sports and Active Living, 2023;5:1054542. frontiersin.org