🔬 How Bloom Calculates Your Cycle

Transparency is a core part of Bloom's privacy-first approach. Here's exactly how every prediction is made.

📅 Cycle Phase Detection

Bloom divides your cycle into four phases based on your configured cycle length:

🌺 Menstruation Days 1 through your period length Active period
🌱 Follicular After period ends until ~2 days before ovulation Estrogen rises, energy building
🌸 Ovulation Ovulation day ± 1 day Egg release, peak fertility
🌙 Luteal After ovulation until next period Progesterone rises, PMS possible

🥚 Ovulation Prediction

Ovulation Day = Cycle Length − 14

This is the calendar method — it assumes the luteal phase (after ovulation) is consistently ~14 days. This works for most people, but individual biology varies.

Accuracy: Calendar-only method is ~75-80% effective for fertility awareness. For higher accuracy, track your basal body temperature and cervical mucus using the Daily Log.

🌿 Fertile Window

Fertile Window = Ovulation Day − 5 through Ovulation Day + 1

Sperm can survive up to 5 days inside the body, and the egg survives ~24 hours after release. This creates a ~6-day window where conception is possible.

🌡️ Symptothermal Method (Advanced)

For higher accuracy, Bloom supports basal body temperature (BBT) and cervical mucus tracking:

  • BBT: Temperature rises ~0.3°C after ovulation due to progesterone. A sustained 3-day rise confirms ovulation occurred.
  • Cervical Mucus: Changes from dry → sticky → creamy → egg-white (peak fertility) → dry after ovulation.
  • Combined: When BBT shift + mucus dry-up both confirm ovulation, accuracy jumps to 99.6%.

Track these daily using the Daily Log page and view patterns on the Trends page.

📊 Cycle Length & Averages

Bloom calculates your average cycle length from the intervals between consecutive period start dates. Only cycles between 15-60 days are included (to filter out data entry errors).

The more periods you log, the more accurate your predictions become. After 3+ cycles, Bloom can detect irregular patterns and alert you.

🔒 What Bloom Does NOT Do

  • No AI prediction: Bloom uses transparent mathematical formulas, not opaque machine learning models.
  • No cloud sync: All calculations happen on the server running Bloom. When self-hosted, no data leaves your device. On a hosted instance, data stays on that server but doesn't sync to third-party services.
  • No medical claims: Bloom is an awareness tool, not a medical device. Always consult healthcare providers for medical decisions.
  • No data selling: Your cycle data is yours alone. See our Privacy Policy.

📚 References

  • Wilcox AJ, Dunson D, Baird DD. "The timing of the 'fertile window' in the menstrual cycle." BMJ. 2000;321(7271):1259-1262.
  • Frank-Herrmann P, et al. "The effectiveness of a fertility awareness based method." Human Reproduction. 2007;22(5):1310-1319.
  • ACOG Practice Bulletin No. 110. "Noncontraceptive Uses of Hormonal Contraceptives." 2010.