MoonstrateUnderstanding your cycle
From Learn about your cycle. General wellness information, not medical advice. Sources.
What happens during your period
Your uterus is lined with tissue called the endometrium, and it has two layers. The deeper base layer stays in place all the time. The top layer is the part that thickens over each cycle, fed by small coiled blood vessels that grow into it, ready in case a pregnancy begins.
When no pregnancy starts, progesterone drops near the end of the cycle, and that drop is the signal for a period. It sets off a controlled, local inflammation in the top layer: immune cells move in, and enzymes break down the tissue that holds it together. The small blood vessels tighten to limit bleeding, and the top layer comes away bit by bit over several days rather than all at once. Your uterus contracts to help move it out. Chemicals called prostaglandins are behind the cramping pain, and people with painful periods tend to have more of them, which is why pain relievers like ibuprofen, which block prostaglandins, help so many people.
What comes out isn't all blood. Two small studies found that, on average, about a third to a half of it is blood, and the rest is fluid and tissue from the lining.
Only a few small studies have measured how much blood a period involves, which is surprisingly little research for something so basic. (There's more on that gap in Why so much of this was never studied.) They put the average at around 30 to 40 mL for the whole period, about two to three tablespoons. But even in one study of just 80 people, amounts ranged from about 5 mL to 169 mL, so it varies widely from person to person, with things like age, birth control, and health conditions all playing a part. There isn't a single normal amount. What matters more is whether your bleeding gets in the way of your life.
Clots. Each small vessel stops bleeding the usual way, with platelets and clotting proteins sealing it. At the same time, the lining makes enzymes that dissolve clots, which is why period blood usually comes out runny. Small clots are common, and bigger ones tend to go with heavier flow. Clots about the size of a quarter or larger are worth mentioning to a clinician, as is bleeding that lasts more than seven days.
Repair. Healing starts quickly, even while you're still bleeding. The base layer and the patches that didn't shed begin covering the surface within days, and then rising estrogen rebuilds the top layer. That's the start of Spring.
If you use hormonal birth control, some of this changes. Combined methods (the pill, patch, and ring) keep the lining thin, and the bleed in the hormone-free days is a withdrawal bleed caused by the hormone drop, not a true period. Some methods can also be taken without the break, which changes the bleeding pattern. A clinician can help you decide what's best for you. The hormonal IUD (intrauterine device) thins the lining where it sits, so bleeding often gets lighter over time or stops, though spotting is common in the first months. Progestin-only methods (the mini-pill, implant, and shot) thin the lining too, and in some people leave fragile small vessels behind, which is why bleeding can become unpredictable. The copper IUD, which has no hormones, often makes periods heavier and crampier, especially in the first year. Local inflammation and extra clot-dissolving activity are the likely reasons, but that isn't settled. There's more on each method in Hormonal methods and your cycle.
What counts as a "normal" cycle
There's no single right number. A typical cycle runs anywhere from about 21 to 35 days (clinicians often use a slightly tighter 24 to 38), and anything in that range is normal.
The famous 28-day cycle is more myth than rule. Only about one in seven people actually run exactly 28 days, and only about one in eight ovulate on the classic "day 14." Your own cycle shifts a little month to month as well, usually by a couple of days either way, and that's normal too. The follicular phase, before ovulation, is the part that flexes most. The luteal phase, after it, stays fairly steady.
What's worth watching is your own pattern over time. If your cycle lengths swing by more than about a week across a year, or regularly fall under 21 or over 35 days, that's worth mentioning to a clinician. Not because something is wrong, but because it's useful information to share. (More on what's worth raising in When something's worth checking.)
Does your cycle follow the moon?
It's an old idea, and it's easy to see where it comes from. The average cycle runs about 29 days, and the moon takes about 29 and a half to go from new to full and back again. The word itself carries the link: "menstruation" comes from the Latin for "monthly," and "month" comes from the moon. Old English called a period monaðblot, "month-blood."
But a similar length isn't the same as keeping time together. When researchers have looked at large groups of people, most found no link between the moon's phase and when periods start, or only a very faint one. One large study of more than 300,000 cycles found a weekly rhythm, with periods starting a little more often on Thursdays and Fridays, that was clearer than any lunar one. And your own cycle shifts by a few days from month to month, so even if it matched the moon one month, it would drift out of step soon after.
The question isn't completely closed. One research group studied long personal period diaries, some kept for many years, and found that some people's cycles lined up with the full or new moon for stretches of time, then drifted apart. The alignment was weaker in recent records, and the group suspects modern night-time lighting. Those studies are small, the people in them volunteered their own diaries, and nobody else has repeated the work yet. That makes it an interesting open question, not something to plan around.
So there's nothing to gain from timing your life, your food, or your fertility to the moon. What does help is learning your own pattern, which is what the Inner Seasons are for.
Common symptoms, and what's behind them
Cramps come from your uterus contracting, driven by natural compounds called prostaglandins. They usually start just before or as bleeding begins, and they tend to peak on the first day or two, when prostaglandin levels are highest. Period pain is very common, and for many people it's more than a nuisance. In one large survey, more than a third of people said their period symptoms kept them from some of their usual activities. Light movement can ease cramps for some people, and anti-inflammatory pain relievers work by dialing those compounds down.
The premenstrual stretch (premenstrual syndrome, or PMS). In the few days before your period, estrogen and progesterone drop sharply. For many people that's when mood dips or low days cluster. The leading explanation is that the brain reacts to the fall itself, more than to how low the levels get. By the time your period starts, hormones have reached their lowest point, and they stay there for a few days before estrogen starts to climb again. Once they've settled there, many people notice their mood lifting over the first days of their period, though that's also when cramps and other period symptoms can be at their worst. There's more on that timing in When the rough days land. Plenty of people feel little or nothing, though, and there's no single "PMS script" your body is supposed to follow, so what you notice in your own logged days tells you more than any general version. The part worth holding onto: this is about your brain's sensitivity to a normal hormone fall, not "imbalanced" or "broken" hormones. Nothing is wrong with you.
Cravings and appetite often pick up in that same late stretch. Real, and different for everyone. Regular meals with some protein and fiber can help keep your energy steady, and if cravings or appetite ever feel out of your control, that's worth raising with a clinician.
Bloating and water retention are commonly reported before a period. The exact mechanism isn't settled, so we'll call it a common experience rather than a rule.
"Brain fog." Some people report it premenstrually. Worth knowing that studies haven't found an actual drop in thinking or memory across the cycle, so if you feel foggy it's real to you, but it isn't a deficit you have to manage around.
Across all of it, everyone is different, and most of this varies from one person, and one month, to the next.
When the rough days land
The name "PMS" points at the days before your period, and for some people that's exactly where the hard days sit. For many others, though, the roughest stretch lands around the period starting: the days right before it through its first days, rather than ending the moment bleeding begins. Part of that may be pain. People with more painful periods tend to report more anxiety and low mood, and in small studies, people with severe cramps felt worse, slept worse, and enjoyed life less on their painful days than on their pain-free ones. Those studies are small, which fits a long pattern of period pain being brushed aside, even by pain researchers. (More in Why so much of this was never studied.) If the first days of your period are hard too, you're not doing anything wrong. That shape is common; it just never got its own acronym.
Plenty of people, meanwhile, have no clear cycle-mood pattern at all. That's just as normal, not a failure to notice something everyone else feels. Stress and sleep often move mood more than the cycle does.
And if a rough patch comes back cycle after cycle and regularly disrupts your life, that has a name, PMDD (premenstrual dysphoric disorder), and it deserves real care. It's treatable, and you don't have to just put up with it.
Talking about it
A lot of people who menstruate grew up learning to keep their periods quiet: hide the pad, don't mention the cramps, push through the bad days. That silence is learned, and it has costs. In one large survey, half the people who had to skip tasks because of their period didn't tell their family why. People who feel more stigma around periods are also less likely to talk about severe pain, even with a doctor, which may be one reason conditions like endometriosis often take years to diagnose.
Talking helps in a simpler way, too. Hearing how someone else's cycle goes is one of the best ways to learn what's common, what's just you, and what might be worth checking. That might be a friend, a sibling, a partner, a parent, a group chat, or a community online: whoever you feel safe with.
If there's no one you can safely talk to right now, that's okay too. What you log in Moonstrate is private, and it's yours to share or keep to yourself. When you're ready to talk to a clinician, your own history is a good place to start.