You Used to Be Able to Set a Watch by This
Body · The Reading Room · 8 min read · Why the timing breaks first, what your hormones are actually doing instead of declining, and what is worth a call
Based on the published research of Nanette Santoro, MD, Professor and E. Stewart Taylor Chair, Division of Reproductive Sciences, University of Colorado Anschutz Medical Campus. Faculty profile · ResearchGate · Endocrine Society · LinkedIn
For twenty-some years your cycle behaved like a fixed appointment. Now it arrives early, arrives late, doubles up, or does not show, and the app guessing at the middle of it knows exactly as much as you do.
The app still says three days. It said that Tuesday too, and the Tuesday before, in the same calm blue font, as if it knows something you don’t.
For most of your adult life this wasn’t a guessing exercise. Twenty-eight days, sometimes twenty-nine, close enough to plan the dentist or a keynote around. Last month it came after sixteen days. The one before took thirty-nine.
None of it is the app’s fault, and none of it means something is wrong. This is irregular periods perimenopause: the ovulation underneath your cycle stops keeping a schedule, months before the bleeding itself stops.
This is a reading of published research. I am not a clinician; the findings belong to the researchers named below, and the plain-English version is mine.
The short answer: cycles get unpredictable in perimenopause because ovulation becomes unpredictable first. When an egg releases on an erratic schedule, or does not release at all, the progesterone that normally follows two weeks later and triggers bleeding on time never arrives in its usual rhythm. A cycle can stretch past six weeks, close in half that time, or repeat itself before you’ve restocked, often years before periods stop for good.¹²
The skim version
- Perimenopause is staged by menstrual bleeding patterns before anything else: a persistent difference of seven days or more between consecutive cycles marks the early transition, and a gap of sixty days or longer marks the late transition, under criteria a research consortium finalized in 2012.¹
- The unpredictability starts with ovulation, not with bleeding. Impaired egg development and irregular ovulation, including what researchers call luteal out-of-phase events, make timing unreliable well before periods stop.²
- Follicle-stimulating hormone and estradiol do not decline in a straight line across the transition. They move unevenly, which is one reason last month’s cycle length predicts so little about this month’s.³
- Mayo Clinic describes the range itself, longer or shorter gaps, lighter or heavier flow, skipped periods, as the ordinary shape of the transition rather than a symptom needing correction on its own.4
- A short list of patterns is worth a same-week call instead of a shrug: periods closer than twenty-one days apart, bleeding beyond a week, bleeding between periods, or any bleeding after twelve full months with none.45
In this article: Why the timing breaks first · Why hormones swing instead of decline · What’s ordinary, and what’s worth a call · The one move · FAQ
Irregular Periods Perimenopause: Why the Timing Breaks First
Because ovulation is the part that goes first, and ovulation is not something you can see or feel happening.
Nanette Santoro, whose research has tracked hormonal change across the menopause transition for decades, describes ovulation becoming unreliable well before menstruation stops: impaired follicle development, atypical estradiol secretion, and cycles her group has termed luteal out-of-phase events, where ovulation lands outside its usual window and produces an irregular cycle around it.² The ovary is still working. It has simply stopped keeping the appointment.
Every period since adolescence has followed roughly the same contract. An egg releases, and about two weeks later, if it isn’t fertilized, progesterone drops and bleeding starts, on schedule, because the whole system was built around that timing. Skip the ovulation, or release the egg early, late, or twice in one stretch, and the two-week countdown either doesn’t start, starts from the wrong day, or starts twice. Whatever bleeding follows inherits all of that. It isn’t choosing to be erratic. It’s downstream of something that already was.
What the research found
The study. Santoro, N. “Perimenopause: From Research to Practice.” Journal of Women’s Health, 2016, 25(4):332–339. A synthesis of the endocrinology of the menopause transition, drawing on the Study of Women’s Health Across the Nation and related cohorts.²
The finding. Ovulatory dysfunction, including impaired folliculogenesis and luteal out-of-phase ovulatory events, underlies the irregular cycles of the transition, alongside estradiol secretion patterns that do not track a simple decline.² The limitation: this is a synthesis of cohort and clinical evidence, not a single trial measuring one woman. It describes the transition on average. It cannot forecast which shape your own next cycle will take.
Why Do Hormones Swing Instead of Just Declining?
Because the ovaries are running out of a resource, not turning down a dial.
It’s tempting to picture perimenopause as estrogen sliding downward in a smooth line until it reaches menopause. John Randolph and colleagues, following women through the Study of Women’s Health Across the Nation, found something messier: follicle-stimulating hormone and estradiol both change unevenly across the transition, and the shape of that change relates to how far a woman actually is from her final period, not to her age on the calendar.³ Some months the ovaries respond close to normally. Other months, less so. Nothing is being turned down at a fixed rate. A supply is running low in a way that produces real month-to-month variation instead of a predictable slope.
That is also why the staging system built for this transition, called STRAW+10, uses your actual bleeding pattern instead of a hormone number to place you. The early transition is marked by cycles that differ from one another by seven days or more, consistently. The late transition starts once a gap reaches sixty days.¹ Both stages are defined by unpredictability itself, which is an unusual thing for a medical criterion to be built on, and also the most honest one available.
If the not-knowing is the part that’s actually exhausting, more than the bleeding itself, that particular kind of tired has its own quiet next step. Start here.
What’s Ordinary Here, and What’s Worth a Call?
Because irregular has a genuinely wide normal range in perimenopause, and a much narrower set of patterns that fall outside it.
Mayo Clinic states the ordinary shape plainly: as ovulation becomes unpredictable, the time between periods may be longer or shorter, flow may run lighter or heavier, and periods get skipped.4 None of that, by itself, is a sign of a problem. It’s the transition doing what the transition does.
A smaller set of patterns is different. The federal Office on Women’s Health is direct about one of them: missing a few periods does not automatically mean perimenopause, and it’s still worth a visit to rule out pregnancy or another cause.5 And once twelve full months have passed with no bleeding at all, any bleeding or spotting after that point needs a same-week call, not a wait-and-see.5
| What you’re noticing | What it usually means |
|---|---|
| Cycles vary by a week or more, month to month | The expected pattern of early perimenopause¹ |
| A period skips entirely for a month or two | Common, more so later in the transition¹ |
| No period for sixty days or longer | Marks late perimenopause. Worth mentioning at your next visit¹ |
| Two periods land less than three weeks apart | Worth a call rather than a shrug4 |
| Bleeding runs past seven days, or soaks through protection within an hour | Worth a call4 |
| Any bleeding after twelve full months with none | Worth a same-week call5 |
The app is guessing. So, most months, is your body.
Definition
Anovulation is a menstrual cycle in which no egg is released. Without ovulation, the progesterone rise that normally arrives on a fixed schedule and triggers a period roughly two weeks later never happens the same way, which is what lets the surrounding cycle stretch, contract, or vanish for a stretch on its own.²
Who else has measured this
Sioban D. Harlow, PhD, Professor Emerita of Epidemiology, University of Michigan School of Public Health. She led the STRAW+10 collaborative that set the seven-day and sixty-day criteria used throughout this article, the closest thing perimenopause has to an official map.¹
U-M faculty profile · Michigan Experts · Population Studies Center · ResearchGate
John F. Randolph Jr., MD, the J Robert Willson Legacy Professor of Obstetrics and Gynecology, University of Michigan Medical School. His analysis of the Study of Women’s Health Across the Nation found that FSH and estradiol change unevenly across the transition rather than declining in a straight line.³
U-M Medical School profile · ResearchGate
Stephanie S. Faubion, MD, MBA, Director of the Mayo Clinic Center for Women’s Health and Medical Director of The Menopause Society. Her clinical guidance shapes how the field advises patients on when ordinary irregularity needs a closer look.
Mayo Clinic Research profile · Mayo Clinic bio · ResearchGate · LinkedIn
The One Move: Stock It Once, Stop Calculating Daily
Five minutes, tonight, in whichever bag actually goes to work with you.
Buy a small zip pouch. Put two days’ worth of whatever you’d reach for, pads, a cup, a spare pair of underwear, inside it. Zip it into the inside pocket of that bag, and leave it there. Not the bag you sometimes carry. The one that’s with you on an ordinary Tuesday.
This doesn’t make the timing predictable. Nothing in this article does that. What it removes is the twice-a-day arithmetic, the checking, the quiet risk calculation you’re running every time you leave the house without knowing. The pouch means the answer to what if it comes today is already sitting in your bag, zipped, done, from tonight onward.
Common Questions
Why are my periods so irregular in perimenopause?
Because ovulation stops keeping a fixed schedule before your period does. Impaired and irregular ovulation, including out-of-phase ovulatory events, changes when progesterone arrives and by extension when bleeding starts, which is why cycles run long, short, doubled, or skipped years before periods stop for good.²
How long does the irregular stretch usually last?
The late stage alone, once gaps reach sixty days or more, typically runs one to three years before the final period.¹ The earlier, more erratic stage that comes before it can run considerably longer than that, and its length varies widely between women.
Is skipping a period the same as being in menopause?
No. Menopause is a single point, confirmed only after twelve full months with no bleeding at all.¹ A skipped period, even several, is part of the transition leading up to that point, not the destination itself.
Can I still get pregnant with irregular periods in perimenopause?
Yes, and it’s worth taking seriously. Ovulation is unpredictable, not absent, until you’ve actually reached the twelve-month mark, so pregnancy remains possible even after a long gap between periods.45
When do irregular periods actually need a doctor’s visit?
Sooner rather than later if periods land less than three weeks apart, bleeding runs past a week, bleeding shows up between periods, or any bleeding appears after twelve full months with none. That last pattern, in particular, is worth a same-week call.45
The one move
Tonight, pack a pouch with two days of supplies and zip it into the bag you actually carry. Five minutes, and the daily guessing gets one item shorter.
If the guessing has spread past your cycle into a longer list of things you’re quietly tracking, that’s worth ten private minutes on one page. Start here.
The bleeding was never the part your calendar could actually predict. The ovulation underneath it was, and now you know why it stopped telling anyone, including you.
Related reading: what the transition does to the midlife brain, why the week before a period got harder first, and the heat that arrives on no schedule either.
On the researchers. Nanette Santoro is a reproductive endocrinologist studying the menopause transition, not a treating clinician for anyone reading this; her review synthesizes cohort and laboratory findings and does not forecast any individual woman’s next cycle. Sioban Harlow is an epidemiologist, now emerita, who helped build the staging criteria used throughout this piece. John Randolph’s SWAN research describes averages across thousands of measured cycles, not a prediction for one. Stephanie Faubion directs clinical guidance for the field and is not offering a diagnosis here.
About the researcher
Nanette Santoro is Professor and E. Stewart Taylor Chair in the Division of Reproductive Sciences, Department of Obstetrics and Gynecology, at the University of Colorado Anschutz Medical Campus. She has studied the endocrinology of reproductive aging for more than two decades, including through the Study of Women’s Health Across the Nation, and her work on ovulatory dysfunction and hormone variability across the menopause transition underlies most of what clinicians now tell patients about why perimenopause feels unpredictable. University of Colorado faculty profile · ResearchGate · PubMed · Endocrine Society · LinkedIn
Disclosure
Informational, not medical advice. See your clinician.
I am not a clinician. I read published research and translate it into plain English.
Blue Leaf Journal has no affiliation with, and no endorsement from, Nanette Santoro, the University of Colorado Anschutz Medical Campus, Sioban D. Harlow, the University of Michigan, John F. Randolph Jr., Stephanie S. Faubion, Mayo Clinic, The Menopause Society, the U.S. Office on Women’s Health, or any journal named here.
No commercial relationship exists with any person, product, or method mentioned.
Every figure above was checked against the cited source on 15 September 2026. If you find something wrong, write to jobrennan@blueleafjournal.com and it will be corrected in place, noted here with the date.
References
1. Harlow, S. D., Gass, M., Hall, J. E., et al., for the STRAW + 10 Collaborative Group. “Executive summary of the Stages of Reproductive Aging Workshop + 10.” Menopause, 2012, 19(4):387–395. PubMed · free full text via PubMed Central
2. Santoro, N. “Perimenopause: From Research to Practice.” Journal of Women’s Health, 2016, 25(4):332–339. journals.sagepub.com
3. Randolph, J. F. Jr., Zheng, H., Sowers, M. R., et al. “Change in Follicle-Stimulating Hormone and Estradiol Across the Menopausal Transition: Effect of Age at the Final Menstrual Period.” Journal of Clinical Endocrinology & Metabolism, 2011, 96(3):746–754. academic.oup.com · PubMed
4. Mayo Clinic. “Perimenopause – Symptoms and causes.” mayoclinic.org
5. Office on Women’s Health, U.S. Department of Health and Human Services. “Menopause symptoms and relief.” womenshealth.gov
Keep reading
Jo Brennan writes the Mind and Load desks at Blue Leaf Journal, and picked up this one because she wanted to understand her own calendar app’s confidence problem.
Written by Jo Brennan for Blue Leaf Journal. Updated: 2026-09-17.
