Under the hood

How Aura Cycles actually works.

Aura is 100% deterministic. Every number you see is computed from your data with a fixed formula. No machine learning, no guesses, no hallucination. Here is the exact math, step by step.

1. Where you are in your cycle

Cycle day
D(t) = (t − slast) + 1
t is today, slast is your most recent period start. Day 1 is the first day of your period. The count is in whole days, so there is no timezone drift.
Your average cycle length
C = round( mean( g1, g2, … ) )
Each gi = si+1 − si is the gap in days between two consecutive period starts. We keep only gaps between 15 and 45 days (anything outside that is an outlier). The mean is rounded to the nearest whole day.
Ovulation (fixed luteal model)
O = C − 14
The luteal phase (after ovulation, before the next period) is physiologically close to 14 days for almost everyone, no matter the total cycle length. So a 28-day cycle ovulates around day 14, and a 25-day cycle ovulates around day 11. This is what lets Aura adapt to your cycle rather than assuming 28 days.

2. Which stretch you are in

The five stretches
1 ≤ D ≤ P→ Rest & Renew (menstrual)
P < D < O − 1→ Follicular Rise (follicular)
O − 1 ≤ D ≤ O + 1→ Ovulation Glow (ovulatory)
O + 1 < D < C − 5→ Luteal Flow (early luteal)
C − 5 ≤ D ≤ C→ Reflect & Rest (late luteal)
P is how many days your period lasts (you tell us). D is your cycle day, C your cycle length, O your ovulation day from step 1.

3. When your next period is due

Next period
snext = slast + C · days
Days until your next period are snext − t. If that is negative, you are that many days overdue.

4. How confident the prediction is

Cycle variability
V = max(g) − min(g)
The spread between your longest and shortest observed cycle gap. Needs at least three period starts to be meaningful.
Confidence
V ≤ 2→ High
2 < V ≤ 5→ Medium
V > 5→ Low
The more consistent your cycles, the more you can trust the projection. We show this openly rather than pretending every cycle is exact.

5. Perimenopause detection

Very early perimenopause (CeMCOR)
nsymptoms ≥ 3 of 9
If you report 3 or more of the 9 characteristic changes (heavier/longer flow, shorter cycles, sore/lumpy breasts, more cramps, mid-sleep waking, night sweats, worse migraines, more mood swings, unexplained weight gain), Aura flags very early perimenopause, even while your cycles are still regular.
Perimenopause (short or irregular cycles)
C ≤ 25 or V ≥ 7
A cycle length of 25 days or fewer is one of CeMCOR's early markers. Cycle-to-cycle variability of 7 or more days is the STRAW+10 definition of the menopause transition. Either one, without 3+ symptoms, flags perimenopause.

A note on the recommendations

The math above is sourced and verifiable. The day-to-day guidance (what to eat, how to train, when to book the big meeting) is general wellness guidance assembled from standard practice and timed to your stretch. It is not medical advice, and it is not tied to a single study. Perimenopause and menopause in particular deserve a clinician's input.

Sources

  1. Prior JC. "How Can I Tell I Am in Perimenopause?" Centre for Menstrual Cycle and Ovulation Research (CeMCOR), University of British Columbia. Source of the "very early perimenopause" criteria: any 3 of 9 characteristic changes while cycles may still be regular.
    cemcor.ubc.ca/resources/how-can-i-tell-i-am-perimenopause
  2. Harlow SD, Gass M, Hall JE, et al. "Executive summary of the Stages of Reproductive Aging Workshop +10 (STRAW+10)." Climacteric, 2012; 15(2): 105–114. Source of the "cycle length varying by 7+ days" definition of the menopause transition.
  3. Lenton EA, Landgren BM, Sexton L. "Normal variation in the length of the luteal phase of the menstrual cycle." British Journal of Obstetrics and Gynaecology, 1984; 91(7): 685–689. Basis for the fixed ~14-day luteal phase used to place ovulation (O = C − 14).
  4. Wilcox AJ, Weinberg CR, Baird DD. "Timing of sexual intercourse in relation to ovulation." New England Journal of Medicine, 1995; 333(23): 1517–1521. Basis for the fertile window (sperm survive ~5 days, the egg ~12–24 hours).