How Thrive Predicts Your Cycle

Thrive uses your personal cycle history to predict future periods, ovulation windows, and phase transitions. We believe in transparency, so here's exactly how it works—and the peer-reviewed research that backs our approach.

The Basics: Your Personal Average

At its core, Thrive calculates your average cycle length from your logged periods. Once you have at least three completed cycles, we use this average to predict when your next period will start. We also calculate your average period duration (menstrual phase length) to estimate when bleeding will end.

This simple approach works well for most people with regular cycles in their mid-reproductive years. However, menstrual cycles naturally vary based on factors like age—and that's where our predictions get smarter.

Age-Adjusted Predictions

If you provide your year of birth (optional), Thrive adjusts its predictions to account for the natural variability at different life stages:

• Adolescents (under 20): Cycles are naturally longer and more variable in the years following menarche. Thrive widens its "normal" range to up to 45 days, so you won't see unnecessary warnings about being "late."
• Mid-reproductive years (20–45): Cycles tend to be most regular during this time. Thrive uses your full cycle history for the most stable predictions.
• Perimenopause (over 45): As menopause approaches, cycles often become shorter, then longer and more irregular. Thrive shifts to using only your last 3 cycles for predictions, allowing the algorithm to adapt quickly to your changing body.

Scientific References

Our approach is grounded in peer-reviewed research and official medical guidelines:

1. Bull, J. R., Rowland, S. P., Scherwitzl, E. B., Scherwitzl, R., Danielsson, K. G., & Harper, J. (2019). "Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles." npj Digital Medicine, 2, 83. doi.org/10.1038/s41746-019-0152-7
   This large-scale study of over 600,000 cycles confirmed that cycle length decreases with age until around 50, then increases. It also demonstrated that cycle variability is highest in adolescents and perimenopausal individuals.
2. American College of Obstetricians and Gynecologists (ACOG). (2015). "Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign." Committee Opinion No. 651. acog.org
   ACOG officially defines the normal cycle range for adolescents as 21–45 days (wider than the adult range of 21–35 days), reflecting the natural irregularity during puberty.
3. Grieger, J. A., & Norman, R. J. (2020). "Menstrual Cycle Length and Patterns in a Global Cohort of Women Using a Mobile Phone App: Retrospective Cohort Study." Journal of Medical Internet Research, 22(6), e17109. doi.org/10.2196/17109
   This study of app-tracked cycles found correlations between BMI, age, and cycle characteristics, supporting the value of personalized predictions.

Your Privacy

Your year of birth is stored securely and is never shared with third parties. It is used solely to improve the accuracy of your cycle predictions within the app. You can choose not to provide it, and Thrive will still work—just with standard prediction windows.