# The Two Clocks Inside the Menstrual Cycle

> **Reproductive biology**
> Published at: https://mariuscomper.uk/treisprezece-la-suta/en/
> Author: Marius Comper

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Pick any menstrual calendar. Day one is the first day of bleeding. Somewhere in the middle, an ovary releases an egg: that moment is called ovulation. What follows it is an almost fixed countdown, about twelve days long. What comes before it has no fixed deadline at all. Across 612,613 real cycles, tracked scientifically, only 13% lasted exactly twenty-eight days, precisely because of this difference.

Source: Bull, Rowland, Scherwitzl et al., 2019, npj Digital Medicine: 612,613 cycles, from 124,648 people, with ovulation confirmed objectively through basal body temperature and luteinising-hormone testing.

## The two ranges

- Before ovulation (follicular): 10.4–26.8 days (swings by 16.4 days across the study's five groups)
- After ovulation (luteal): 8.0–12.9 days (swings by just 4.9 days)

## Twenty-Eight Days Is Just a Rounded Average

| Cycle length | Of 612,613 cycles | Before ovulation (days) | After ovulation (days) |
| --- | --- | --- | --- |
| 15–20 days | 0.6% | 10.4 | 8.0 |
| 21–24 days | 8% | 12.4 | 11.0 |
| 25–30 days | 65% | 15.2 | 12.4 |
| 31–35 days | 19% | 19.5 | 12.9 |
| 36–50 days | 7% | 26.8 | 12.9 |

The five groups cover 99.1% of the 612,613 cycles. The remaining 0.9% fell outside the study's tabulated 15–50 day range. A cycle of exactly twenty-eight days averaged 15.4 days before ovulation and 12.6 days after.

## The Part That Doesn't Move

After ovulation, the released egg leaves behind a small, temporary structure: the corpus luteum. The corpus luteum produces progesterone, the hormone that prepares the uterus for a possible pregnancy. It runs on an almost fixed clock: without a pregnancy signal, it shuts down around twelve days after ovulation. That's why the part after ovulation stays so tight in the data: its average moves from 8.0 to 12.9 days across all five groups, and its standard deviation stays between 2.1 and 2.8 days throughout.

## The Part That Can Run Late

The part before ovulation has no clock that strict. Each month, the ovary starts with a batch of immature follicles, and only one has to reach full maturity before ovulation can happen. This part's average climbs from 10.4 to 26.8 days across the same five groups, a shift more than three times larger than the part after ovulation.

## The Age Effect, Between Twenty-Five and Forty-Five

Total cycle length falls slightly with age: the observed decline between twenty-five and forty-five was 2.9 days in total, at an average slope of about 0.18 days per year. The part before ovulation falls at a similar pace, 3.2 days over the same span. The part after ovulation stays essentially unchanged.

## Who Was Tracked in This Study

The data come from people using a fertility-awareness app, with ovulation confirmed objectively through basal body temperature and luteinising-hormone testing. The population isn't a random sample of everyone who menstruates: they are reproductive-age adults who had already chosen to track their cycle with a digital tool. The study excludes adolescents at the start of their reproductive lives and people nearing menopause.

Full source: Bull JR, Rowland SP, Scherwitzl EB, Scherwitzl R, Danielsson KG, Harper J. "Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles." npj Digital Medicine 2, 83 (2019). Full text: https://pmc.ncbi.nlm.nih.gov/articles/PMC6710244/