# Why your predictions shift

> Your app is not broken. Your body is just being a body.

Category: Cycle basics · 4 min read · Written by the Splash team · Sources checked October 2026
URL: https://splash.care/learn/cycle-basics/why-predictions-shift

## Quick answer

- Predictions are averages based on your past cycles.
- They update as you log, and can move with stress, illness or travel.
- More data, especially LH tests or temperature, improves the guess.
- Predictions are not contraception.

You open the app and your predicted period has moved by three days. Rude. But it is usually a sign the app is learning from you, and bodies simply do not run to a timetable.

## How predictions work

Splash looks at your recent cycle lengths, works out an average, then projects forward. [Ovulation](https://splash.care/learn/glossary/ovulation) is estimated by counting back from your expected period, because the [Luteal phase](https://splash.care/learn/glossary/luteal-phase) is usually steadier than the [Follicular phase](https://splash.care/learn/glossary/follicular-phase). Steadier is not the same as fixed though. Real-world data show luteal phases of roughly 7 to 17 days.

## Why they move

- **Natural variation:** in a large real-world study, the same person's [cycle length](https://splash.care/learn/glossary/cycle-length) varied from month to month, and [ovulation day](https://splash.care/tools/ovulation-calculator) varied a lot between people.
- **Life events:** [stress](https://splash.care/learn/cycle-basics/stress-travel-illness), illness, travel and [sleep](https://splash.care/learn/movement-sleep/sleep-cycle) changes can delay [ovulation](https://splash.care/learn/phases/ovulation), which delays your period.
- **New data:** each new period you log updates your average.
- **Few cycles logged:** with only one or two cycles, the app has little to go on.

**30%** of women had a fertile window entirely within days 10 to 17 of their cycle in one study of 696 cycles. For most, it fell earlier or later (Wilcox, BMJ 2000).

That study (221 women trying for a baby, with regular cycles) also found that every day from day 6 to day 21 had at least a 10% chance of being in the [fertile window](https://splash.care/learn/glossary/fertile-window). In plain words: even regular cycles are hard to predict from the calendar.

> **Splash says:** adding LH test results or temperatures gives the app real ovulation evidence instead of a guess. Note a reason on any odd cycle (illness, travel, new medicine) so it does not skew your averages.

> Because predictions are estimates, do not rely on them to avoid pregnancy. If you do not want to be pregnant, use contraception that is designed for it. Fertility awareness methods need proper teaching and careful daily use.

**Myth:** If the app says it is a low-fertility day, you cannot get pregnant.

**Fact:** Calendar predictions cannot know exactly when you ovulate this cycle. There are no guaranteed safe days.

## Sources

1. [Bull et al. (2019) Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles, npj Digital Medicine](https://pmc.ncbi.nlm.nih.gov/articles/PMC6710244/)
2. [Wilcox et al. (2000) The timing of the "fertile window" in the menstrual cycle, BMJ](https://pubmed.ncbi.nlm.nih.gov/11082086/)
3. [NHS: Natural family planning (fertility awareness)](https://www.nhs.uk/contraception/methods-of-contraception/natural-family-planning/)
4. [NHS: Periods and fertility in the menstrual cycle](https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/)

This is general information, not medical advice. Splash can’t diagnose. If you’re worried, talk to a GP, pharmacist or sexual health clinic. In an emergency call 999 (UK) or 911 (US).