Reproduction · Process
The menstrual cycle
Two organs, one repeating month. An egg matures in an ovary while a lining is built in the uterus — and the two are timed against each other.
Start here
Day 14 is not when the egg is released. It is when the egg is released in a 28-day cycle.
Ordinary cycles run anywhere from about 21 days to about 35, and the same person's cycles vary between themselves. So the day the egg is released cannot be the same number for everybody.
A person's cycle is 32 days long. Roughly when is the egg released?
Around day 18 — and you get there by counting backwards. The gap between the egg being released and the next period is close to a fortnight in almost everyone. The part that stretches or shrinks is the building phase before release. So a 21-day cycle releases around day 7, a 28-day cycle around day 14, a 35-day cycle around day 21, and the famous number turns out to be one case rather than the rule.
Set a cycle · walk the days
One month, two organs
1 of 3 lengths tried
Choose a cycle length, then move through the days. Watch where the release day lands each time you change the length.
Cycle length
Day 1
Period
In an ovary
Nothing has been released. Egg cells are present, and one of them will begin the run of maturing that ends in this cycle’s release.
In the uterus
The lining built during the last cycle is breaking down and passing out through the vagina. This is the bleeding, and it is why day 1 is easy to identify.
Now change the cycle length and watch the release marker move.
The cycle, in four events
It ends where it starts, which is why it is drawn as a circle.
The lining breaks down · Days 1–5 — the period
The lining built during the last cycle is no longer being held, so it breaks down and passes out through the vagina. By convention the first day of bleeding is day 1 of the cycle, which is simply the easiest day to notice.
Building, and one egg maturing · From about day 5 to release
The uterus lining thickens again with tissue and blood vessels, while in one ovary a single egg cell finishes maturing. This is the stretchy part of the cycle: it is longer in a long cycle and shorter in a short one.
Release · About a fortnight before the next period
The mature egg cell leaves the ovary and is drawn into the oviduct, where it can be fertilised for roughly a day. In a 28-day cycle that falls near day 14; in a 21-day cycle nearer day 7; in a 35-day cycle nearer day 21.
Held ready · The fortnight after release
The lining is at its thickest and is kept that way. If a fertilised egg implants, it stays and the cycle stops here. If none does, it breaks down — and that is event 1 of the next cycle.
Key fact
The menstrual cycle is two things happening at once in two different organs: one egg cell maturing and being released from an ovary, and the lining of the uterus being built up and then broken down. Day 1 is the first day of bleeding — a place to start counting, not the start of the process.
Think again
“The cycle is 28 days and the egg comes out on day 14.”
Twenty-eight is an average of many people's cycles, and averages are not rules — no more than the average family has 1.7 children describes an actual family. Cycles from about 21 to about 35 days are entirely ordinary, and one person's own cycles vary from month to month, which is the part most often left out. What is fairly steady is the other end: the gap from release to the next period is close to a fortnight in almost everyone. That is why the arithmetic runs backwards from the end of the cycle rather than forwards from day 1, and it is why a calendar alone predicts this badly. If you take one number away from this lesson, take the range rather than the average.
“A period is the unfertilised egg leaving the body.”
It is not, and the sizes make that obvious once you have them. An egg cell is about 0.1 mm across — barely visible, and nothing you could ever see leave. What leaves is the lining of the uterus: the layer of tissue and blood vessels that was built up over the previous few weeks, breaking down and passing out through the vagina because no fertilised egg implanted in it. The egg itself simply breaks down where it is, unnoticed. Nor is any of it waste the body was storing up: it is a lining that was built for a job, kept ready for about a fortnight, and cleared when the job did not arrive. Then it is built again. The uterus is not disposing of something; it is resetting.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Use the rule
Someone’s cycles are 24 days long. Roughly which day is the egg released?
Rung 2 · The one that catches people
What leaves the body during a period?
Rung 3 · Describe one whole cycle
Describe what happens in the ovary and in the uterus across one complete cycle. Say what event day 1 is counted from, and make clear which two things are happening at the same time.
Rung 4 · Take it somewhere new
Two people have cycles of 26 and 34 days. Working from day 1, say roughly when each releases an egg, explain why the eight-day difference is not shared evenly across the cycle, and give one reason a calendar still cannot predict the day reliably.
Key note
In the ovaries, one egg cell finishes maturing and is released roughly a fortnight before the next period. In the uterus, the lining thickens ready to receive a fertilised egg, is held for about a fortnight, and breaks down if none implants — which is the period, and day 1 of the next cycle. Cycle lengths of about 21 to 35 days are ordinary and vary within one person; 28 days is an average, not a rule.
Going further
Where did 28 come from? It is close to the average of a large number of cycles, it is close to a lunar month, and those two coincidences together gave it a grip on the subject that the evidence never supported. When researchers began tracking large numbers of real cycles — first with daily temperature and hormone tests, more recently with app data from millions of users — the picture that came back was a spread, not a number: most cycles somewhere in the twenties or low thirties, the same person often varying by several days between one cycle and the next, and the day of release scattered accordingly. The steadier interval is the one after release. This is a good example of a general problem with averages in biology: the average is a real fact about the population and can still describe almost nobody in it. Anything personal about your own cycle is a question for a school nurse, doctor or pharmacist rather than a calculation.
Before this lesson
- This follows on from Gametes and fertilisation.
Connects to
At GCSE this becomes
- The four hormones that control the cycle, the graphs that go with them, and how contraception and fertility treatment work on that control.
Where to next
Ask Mr Badmus AI
Want to check why the counting runs backwards?
If any of this is about you or someone you know, talk to someone you trust — a parent or carer, a teacher, your school nurse, a pharmacist or your GP. Out of school hours: Childline — 0800 1111, free and confidential.
Cycle lengths and the day of release are typical values: about 21 to 35 days is ordinary, and one person's cycles vary between themselves. Hormone names and hormone graphs are deliberately outside Key Stage 3 and are met at GCSE. This lesson covers the biology; anything personal belongs with your school nurse, doctor or a trusted adult, and relationships and consent are covered in RSE and PSHE.
Lesson content © MrBadmusAI.