The Menstrual Cycle Is a Vital Sign. Almost Nobody Was Taught to Read It.
Your cycle is your fifth vital sign.
My periods have been heavy, painful, absent, present, heavy again, and now quiet and unremarkable, all within the same body, over the better part of thirty years. Different doctors saw each phase as its own isolated event. None of them were.
I got my first period in late primary school, and it was heavy and painful from the start. I didn't know it then, but I understand it now as an early signal: a body already showing signs of progesterone insufficiency and inflammation, running on a diet that was heavy in refined carbohydrates and light on almost everything else, in a kid who was also depressed. By my late teens I'd developed an eating disorder, and my period simply left. It came back, briefly, the moment I started eating a little more, then disappeared again as the restricting resumed. That pattern, restriction, then adequate eating, then restriction again, ran through most of my twenties, and my cycle tracked it almost exactly. When I ate enough, it showed up. When I didn't, it didn't.
I want to come back to my own story later in this piece, because I think the shape of it says something a lot of women will recognise in their own history. But first I want to explain why your cycle is capable of telling this kind of story at all, and why almost nobody has ever shown you how to read it.
What Actually Counts As A Vital Sign
A vital sign is a measurement that reflects the state of the whole system, not just the part it's taken from. Blood pressure isn't only about your arteries. Temperature isn't only about your skin. Both are read as proxies for something bigger: how well the body, as a whole, is coping with its current conditions.
In 2015, the American College of Obstetricians and Gynecologists formally recommended that clinicians treat the menstrual cycle the same way, asking about cycle length and pattern at every routine visit, the same way a nurse asks about blood pressure. The opinion set a statistically derived normal range and recommended a diagnostic workup for cycles that fall outside it. For adults, a separate international nomenclature system puts the normal range at 24 to 38 days, based on population data from women aged 18 to 45.
The reasoning behind treating the cycle this way is straightforward once you see it. The reproductive system is metabolically expensive to run, and it sits downstream of nearly everything else in the body: thyroid function, nervous system regulation, inflammatory load, nutrient status, and the amount of energy actually available for the body to spend on anything beyond survival. When one of those systems is under strain, the cycle is often one of the first places that shows up, well before it shows up anywhere else measurable. That's what makes it a vital sign rather than a reproductive detail. It's a monthly readout of how the rest of the system is doing.
Ten years after that recommendation, most women have still never had a doctor ask about their cycle that way. It gets treated as a fertility question, or a nuisance to manage, rather than as information about anything else going on in the body.
The Difference Between Tracking And Reading
Somewhere in the last decade, tracking a cycle became genuinely normal. Millions of women now log their periods in an app. That's a real improvement on having no record at all. But tracking and reading are not the same activity, and most of the tools built for tracking were never designed to help anyone read.
Tracking answers a narrow question: when is my next period likely to arrive? Most apps do this by assuming a textbook 28-day cycle with ovulation on day fourteen, which is why research testing app predictions against actual, hormonally confirmed ovulation found the apps were accurate no better than 21% of the time, with the majority of predictions landing two to nine days early. The apps aren't reading your cycle. They're pattern-matching it against an average that a large proportion of real cycles don't actually follow.
Reading is a different question entirely: what is this pattern telling me about what my body is currently able to do? A cycle that arrives late, or early, or with unusual bleeding, or with pain that derails your week, isn't a scheduling inconvenience.
It's a message, and most women have never been taught the alphabet it's written in.
Why This Gap Exists
Part of the gap is structural. A standard consultation doesn't have room for a proper cycle history, and reproductive health training in general medicine tends to focus on contraception, screening, and pathology rather than on interpreting the ordinary variation of a healthy or struggling cycle. Part of it is cultural: period pain and irregularity have been treated as background noise for so long that women routinely go an average of nearly seven years between the start of symptoms and an endometriosis diagnosis, and that delay lengthens substantially further when a patient's early reports of pain are dismissed rather than investigated. And part of it is that the tools women have been handed, the calendar apps, the "just track it" advice, were built to predict, not to explain.
I see the consequences of this gap constantly in clinic. Women who've had cycles as short as twelve to fifteen days for years, and no GP or endocrinologist has ever told them that isn't normal. Women who've been on the pill since fifteen for "irregular periods" and have genuinely never seen their own unmedicated cycle as an adult. Women who assume their pain is simply what periods are, because nobody offered a competing explanation.
The Energy Budget Underneath It
Underneath most of what shows up in a cycle sits the same piece of biology: a system in the brain that is constantly running an energy audit, and a menstrual cycle is one of the more expensive things it can choose to fund.
The key player is an enzyme called AMPK, short for AMP-activated protein kinase, and it functions as a fuel gauge inside cells. When cellular energy runs low, AMPK activates. More than 95% of the neurons that release GnRH, the hormone that drives the entire reproductive cascade, carry this fuel gauge, and when it switches on, it suppresses the kisspeptin neurons that act as the gatekeepers between the brain and the reproductive axis. Kisspeptin is what tells GnRH neurons to fire in the pulsed rhythm that ovulation depends on. Silence the gatekeeper, and that rhythm slows, becomes erratic, or stops.
This is not a flaw in the system. It's the system doing exactly what it's built to do: read the available energy, and decide whether this is a sound moment to fund a metabolically expensive process that isn't required for immediate survival. AMPK isn't the only input reporting scarcity to this circuit either. Psychological and physiological stress work on an overlapping pathway, and functional hypothalamic amenorrhea, where the cycle shuts down in response to inadequate energy intake, psychosocial stress, or the combination of the two, is one of the best-documented examples of this in the clinical literature. Even well short of full amenorrhea, sustained energy deficit is associated with a shortened luteal phase and reduced progesterone production, often long before a cycle stops arriving altogether. Whether the scarcity is caloric or perceived as threat, the hypothalamus appears to be running some version of the same calculation: is this a safe moment to spend resources on reproduction.
That calculation is the thread running through nearly everything below.
Five Signals, And What Each One Is Actually Saying
Long or irregular cycles. When cycles stretch well past 38 days, arrive unpredictably, or disappear for months, the most common underlying story is that ovulation isn't happening consistently, or isn't happening at all. That can be driven by the energy-availability picture above, by thyroid dysfunction, by PCOS and its effect on the LH-to-FSH ratio, or by a combination. What it's signalling, in most cases, is that the brain isn't currently getting a reliable "safe to proceed" signal.
Short cycles. Cycles consistently under 24 days deserve as much attention as long ones, though they get far less. A short cycle usually means the follicular phase has shortened, ovulation is arriving earlier than it should, or the luteal phase itself is too brief to sustain adequate progesterone. A luteal phase of ten days or less is the clinical threshold for a luteal phase defect, and it's frequently linked to declining ovarian reserve, thyroid imbalance, or the same energy and stress inputs driving the long-cycle pattern above, just showing up as an earlier exit rather than a delayed start. I've had clients arrive in clinic with cycles of twelve to fifteen days who had genuinely never been told this wasn't normal, by a GP or an endocrinologist. It is not normal. It's worth investigating.
Painful periods. Primary dysmenorrhoea is driven substantially by prostaglandins, compounds released from the uterine lining that trigger the muscular contractions of a period. Higher prostaglandin levels are directly correlated with more severe pain, because they don't just trigger contraction, they also constrict blood vessels in the uterine muscle, producing localised ischaemia, the same basic mechanism behind the pain of a cramping muscle starved of blood flow elsewhere in the body. Some prostaglandin production is a normal part of every period. Pain severe enough to derail your week is not simply "a low pain threshold." It's a sign of an inflammatory load that's worth investigating rather than managing with analgesics indefinitely, and it's also one of the clearest early warning signs for endometriosis and adenomyosis, conditions that take years to diagnose partly because this exact signal gets dismissed so routinely.
Spotting before a period. Reliable premenstrual spotting is usually a sign the endometrium isn't being adequately supported by progesterone in the run-up to menstruation. Without sufficient progesterone to hold the lining in place, it can begin shedding unevenly before the main event, which is consistent with the corpus luteum, the temporary structure that produces progesterone after ovulation, not producing quite enough, or not for quite long enough. This is often the same underlying picture as a short luteal phase, just noticed differently.
PMS and PMDD. Premenstrual mood symptoms are frequently dismissed as "just hormones," which is both true and unhelpful as an explanation. PMDD, the more severe end of this spectrum, affects roughly three to eight percent of women, a prevalence similar to generalised anxiety disorder, and it appears to be less a matter of low progesterone in absolute terms and more a matter of altered sensitivity in the brain's GABA-A receptors to allopregnanolone, the calming metabolite progesterone converts into. Some women's nervous systems respond to the ordinary premenstrual dip in this metabolite as a much larger event than it should be. That's a nervous system signal riding on a hormonal one, not "just" either.
This Was My Body Too
I recognise every one of those five patterns somewhere in my own history, which is part of why I wanted to write about them together rather than one at a time.
The heavy, painful periods I had from the very start were, I now think, an early sign of both inflammation and insufficient progesterone, in a body that was under-fed and under a load of undiagnosed depression at the same time. When the eating disorder took hold in my late teens, my cycle did exactly what the energy-availability research would predict: it stopped, then returned the moment I started eating even a little more. Through my twenties, it moved in and out with my eating in a way I could have set a clock to.
Oddly, staying consistent with exercise seemed to help keep my cycle present, even during periods when my eating wasn't where it needed to be. I don't have a confirmed explanation for that, and I want to be honest that this next part is a hypothesis rather than something I can point to a study for. I carry MTHFR and eNOS gene variants, and I've had poor circulation for as long as I can remember, always the person with cold hands regardless of the room. I've wondered whether the benefit I felt from consistent movement had less to do with fitness itself and more to do with blood flow to the ovaries. It's a pattern I hold loosely, not a claim.
In my late twenties and early thirties, running a business under extreme and sustained stress, my period nearly disappeared again, down to two or three times a year, and I doubt I was ovulating in most of the cycles I did have. This is the part of my own story I think is most worth naming clearly: my food and my exercise were consistent through that period. It wasn't a caloric story this time. My body was still struggling for energy, because the stress itself was drawing on the same budget.
Then I closed the business, and the external pressure genuinely eased. My period came back, but not gently. It came back with the heaviest bleeding, the worst pain, and the most swollen, fibrocystic breasts I'd had in my life. It took me a while to understand that removing the external pressure hadn't automatically resolved the internal state my nervous system had been running in for years. It was only once I started doing deliberate nervous system work that things properly calmed, and my cycle settled into something close to a textbook 28 days, with light bleeding and very little pain.
Even after that, before I became pregnant, my cycle kept functioning as an early warning system. If I'd been overdoing things, work, sleep, stress, I'd know before I consciously registered it, because my next cycle would come with more pain, a longer gap, and heavier bleeding. I stopped needing to guess how much I had in the tank. My cycle was already telling me.
What To Actually Do With This
None of the five patterns above are meant to be diagnostic on their own, and I'm not suggesting you should be able to self-diagnose from a blog post. What I am suggesting is that a persistent pattern, not one unusual month, but a repeated signal over several cycles, is worth investigating rather than living with or medicating over indefinitely.
The most useful next step is usually proper cycle-timed bloodwork: oestradiol, LH, and FSH early in the cycle, progesterone roughly five to seven days after ovulation rather than on a fixed calendar day, alongside thyroid and inflammatory markers. Basal body temperature charting adds something a single blood draw can't: a picture of the whole pattern over time, including whether ovulation happened at all and whether the rise afterward held for long enough.
I put together a full guide that walks through exactly this: when in your cycle to test each hormone, what an optimal range looks like rather than just a technically "normal" one, and how to read the common patterns on your own results, the same patterns I've spent years learning to read in clinic. It also covers the rest of a standard blood panel, so you're not left guessing at the sex hormones section in isolation from everything else your body is telling you.
Get the guide, How to Interpret Your Blood Test Results, here →
Track To Predict. Read To Understand.
Tracking your period tells you when to expect it. Reading it tells you something closer to the truth: what your body has had the resources to do, month after month, given everything it's currently carrying.
Neither irregular cycles, short cycles, pain, spotting, nor premenstrual symptoms are proof that something is broken. In almost every case I see, they're proof of a system responding sensibly to real constraints, energy, inflammation, stress, nutrient status, and doing so in a way that's actually possible to investigate and change. That was true of my own cycle for twenty years, and it's true of nearly every pattern I see in clinic.
If any of these five patterns sound familiar, and you'd like a proper look at what's actually driving them, I work with women one-on-one to look at the whole picture: hormones, nutrient status, gut health, and the nervous system underneath it all, rather than any one piece in isolation.