There's a number that's been in every health textbook, every doctor's office poster, and every period tracker for at least a century: the menstrual cycle is 28 days. It's not. It's been wrong for a long time, and it's still wrong.
Where the number came from.
The "28-day cycle" is an artifact of two things: it's the median length from early 20th-century studies, and it's close to the lunar cycle, which was poetically convenient. Both are real. Neither is prescriptive.
Recent large-scale studies put the actual range of healthy cycle lengths at 21 to 35 days, with significant variation between people — and from cycle to cycle in the same person.
What "normal" actually looks like.
- The median is closer to 29 days, not 28.
- About 13% of cycles fall outside the 21–35 day "textbook normal" range — and most of those people are perfectly healthy.
- The average cycle varies by ±2–3 days even in someone with very regular cycles.
- Cycles tend to be longer in your teens and early 20s, and gradually shorten as you age.
What this means for tracking.
If you've been tracking and noticing your cycle isn't 28 days, that's not a problem. It's just a real number. TeenCycle uses your own past cycles to predict your next one — not a textbook average. That's why predictions get more accurate the longer you log, and why your predictions might look different from a friend's.
The point isn't to match the chart. It's to understand your chart.
For more on what to expect as you build your own dataset, see the first-year field guide.
