The Menstrual Phase, Explained: What Is Actually Happening While You Bleed

The menstrual phase is the bleeding itself — the womb lining coming away — and it is where the count starts. Day one is the first day of full flow, not the spotting that sometimes arrives the evening before, and every cycle number on this site is built from that single date. Alongside the bleeding, the NICHD names a familiar list as ordinary company: cramping in the belly or pelvis, lower back pain, bloating and sore breasts, food cravings, mood swings and irritability, headache and fatigue. Most of that is unremarkable. Some of it has thresholds worth acting on, and those are quoted further down rather than left as "see someone if worried".

This page does three things: it settles what counts as day one, it says what the phase is and what commonly comes with it, and it hands over the lines the NHS draws between an ordinary period and one worth an appointment.

Day one, and why one date carries everything

A cycle is counted from the first day of one period to the day before the next starts. That means your cycle length, your average, how much you swing around that average and every estimated date any calculator gives you are all built on top of one entry — and if that entry moves, all of them move with it.

The rule matters less than using the same one every month. If your own pattern is two days of brown spotting and then flow, pick one of them, write down which, and stay with it.

What the phase is called, and what that tells you

There are two naming schemes in use and it is worth being able to read both, because two apps can show you two different phase names on the same Tuesday without either being wrong.

The older four-stage description, set out in a 2015 review of cycle physiology, runs: the menstrual phase, then the proliferative phase — also known as the follicular phase — then mid-cycle, during which ovulation occurs, then the secretory phase, also known as the luteal phase. The two-phase scheme in wider use among physiologists splits the cycle at ovulation instead, and counts your bleeding days as the opening of the follicular phase rather than giving them a name of their own.

The names carry the physiology. Proliferative is what the lining does after a period: it is built back up. The menstrual phase is the other end of that — the lining that was built during the previous cycle coming away. What the phase is not is a pause. The next cycle's first half is already running underneath the bleeding, which is why the follicular phase overlaps it in the two-phase scheme.

Two things this page will not tell you, because none of the publishers it stands on says them. What the hormone levels are doing hour by hour through these days, and what menstrual flow is made of. Both are answerable, and neither is answerable from the sources listed at the bottom of this page, so they are left to a source that carries them.

What commonly comes with it

The NICHD's list is short and worth having in full, because most of what people quietly wonder about is on it. The primary sign of menstruation is bleeding from the vagina. The additional symptoms it names are abdominal or pelvic cramping, lower back pain, bloating and sore breasts, food cravings, mood swings and irritability, headache, and fatigue.

That list is descriptive, not prescriptive. It is a record of what commonly turns up, not a schedule you should expect to keep, and plenty of people get almost none of it. The useful thing to do with it is to notice which items are yours — if headache lands on day one for you every month, that is your pattern and worth writing down. It is not a plan, and nobody should be telling you what to eat or how to train because of where you are in a cycle.

How long it lasts, and what the numbers say

Two to seven days, once periods have settled — that is the NHS's own range, given on its page about periods starting. It is a wide range on purpose, and a two-day period and a six-day period are both ordinary rather than one of them being a sign of something.

The far end is the number worth carrying, because it is the one that turns into an action: periods lasting more than 7 days appear on the NHS's list of signs of heavy periods. Seven days is the point at which length itself is worth raising, rather than a limit anybody has failed.

Cycle length is better measured. Across 612,613 tracked ovulatory cycles the mean was 29.3 days and only 13% were exactly 28 — and the same dataset found the average woman's own cycle length moving by about 2.6 days from one month to the next. The NHS puts the common range at every 21 to 35 days, and the NICHD says the average cycle is 28 days while varying between women and from month to month, with a range of 21 to 45 days in teenagers and 21 to 35 for most adults.

Which is to say: the date a calculator prints is the middle of a window, and the window is the answer. The period calculator estimates your next few periods from your last start date and your usual cycle length and prints the range next to each one. The cycle phase calculator does the same job for where you are today, and it says out loud that a phase boundary read off a calendar is an estimate rather than an event it can see.

Ordinary, and worth a GP's time

Two of the commonest things about a period have thresholds published against them, and a threshold is worth more than reassurance because you can act on it.

Pain. The NHS says period pain happens when the womb tightens during your period, that it is often a normal part of the cycle, that it usually lasts up to 3 days and sometimes spreads to the back and thighs. It says to see a GP if your periods become more painful, heavier or irregular, or if the pain stops you doing your usual daily activities — and to ask for an urgent GP appointment or help from NHS 111 if the pain is severe or worse than usual and painkillers have not helped.

Bleeding. The NHS says you may have heavy periods if you need to change your pad or tampon every 1 to 2 hours, empty a menstrual cup more often than is recommended, need to use 2 types of period product together, have periods lasting more than 7 days, pass blood clots larger than about 2.5cm — the size of a 10p coin — bleed through to your clothes or bedding, avoid daily activities or take time off work because of your periods, or feel tired or short of breath a lot.

Neither list is a diagnosis and this page is not making one. They are the descriptions a clinician recognises, which is exactly what makes them worth quoting: an appointment that starts with "I change a pad every hour and it has been eight days" is a different appointment from one that starts with "my periods feel heavy". The two deeper pieces are what is normal and what is not about period pain and what actually counts as heavy, and both carry the rest of what the NHS says.

What to do with day one once you have it

Write it down on the day. That is the entire discipline, and it is the entry people forget, because on day one you have other things on your mind. A phone note works. So does the printable period tracker, which is a sheet of paper: it asks nothing of you, stores nothing anywhere and cannot be synced, sold or breached.

Three or four start dates in and the numbers start meaning something. Twelve and you can see your own year — how much you actually move, which months went long, whether the pattern has changed. That record is the thing a five-minute appointment can be built on, and it is worth more than any prediction printed on top of it.