The basics
Everything runs through ovulation
3 min read
Here is the physiology in one paragraph. Each cycle, estrogen from a developing ovarian follicle thickens the endometrium — the lining of the uterus. Ovulation releases the egg, and the structure left behind, the corpus luteum, produces progesterone, which matures the lining and holds it in place ready for implantation. If nothing implants, the corpus luteum stops working, progesterone falls, the lining can no longer be maintained, and it sheds. That shedding is the period.
Read that again for what it actually says, because it is the most important sentence on this site:
A menstrual period is progesterone withdrawal bleeding. Progesterone comes only from the corpus luteum. The corpus luteum exists only after ovulation. So a true period only ever happens about two weeks after an ovulation — no ovulation, no period.
That is not a technicality. It means the bleeding is not the event; the bleeding is the receipt for an event that happened a fortnight earlier. And it means bleeding that was not preceded by an ovulation is not menstruation at all — it is estrogen breakthrough or estrogen withdrawal bleeding, which looks similar from the outside and is a different thing underneath.
Three consequences follow, and between them they explain most of what people find confusing:
A period is the end of a cycle, not the beginning of one. We number cycle days from the first day of bleeding purely because it is the one event you can date reliably. Physiologically the bleeding is the last act, not the first.
When ovulation is irregular, bleeding is irregular. When ovulation does not happen at all, there is no progesterone, so the lining keeps thickening under estrogen alone until it breaks down unpredictably — which is why anovulatory bleeding can be both erratic and very heavy, and why prolonged anovulation carries a risk of endometrial hyperplasia. This one mechanism accounts for a large share of unpredictable bleeding in adolescence, in perimenopause, in PCOS and in thyroid disease.
A problem with your menstrual cycle or your menstrual period is usually a symptom, not a diagnosis.
Too heavy, too long. The cause is more often the uterus itself, its lining, or how your blood clots.
Irregular, infrequent or absent. The cause is more often ovulation.
Either way, the underlying disorder may involve ovulation, the uterus, or another reproductive or systemic condition. It has a cause, and causes can be found.
The two halves of the cycle
The first half — from the start of bleeding until ovulation — is the follicular phase. Its length varies considerably, both between people and from month to month in the same person. The second half — ovulation to the next period — is the luteal phase, and it is far more consistent, typically around two weeks.
That asymmetry is the practical point. When a cycle runs long or short, it is nearly always the first half moving. Which is why "day 14 is ovulation" holds only for a textbook 28-day cycle, and why counting backwards from the next expected period estimates ovulation better than counting forwards from the last one.
Every abnormality, properly named. Period problems and cycle problems have specific medical names, and the terminology was substantially revised in 2011 and 2018 — so much of the older vocabulary you may have been given is no longer standard. The full reference page defines every term, in plain language or clinical terminology, and says which findings need evaluation rather than reassurance.