Period calendar: what you see on the screen
- Cycle · this week — a swipeable ribbon of days, a line such as “Fertile window · 3 days to ovulation,” and the Started/Ended button.
- Today’s card — “Cycle day 12,” the phase name and two blocks, “In your body” and “Tip.”
- The full calendar — bright drops for days you marked, pale hatched drops for the forecast, and their own colors for fertile days, ovulation, a delay and intimacy. Tap a day to see what it is: an ordinary day shows just “Cycle day 12.”; an estimated fertile day or ovulation day says it’s “Calculated from your period dates.” Below it, stats by month or year: the period days and intimacy you marked, plus fertile days and ovulations as the calendar estimates them.
- Settings — cycle and period length (“By your own entries it’s 30 days — that’s what we forecast from”) and an optional question about hormonal birth control or breastfeeding (today only the birth-control answer changes the cards).
How the cycle tracker predicts your next period and ovulation
- Next period start = start of your last period + your average cycle length, shown as a range: ±2 days, or ±4 days when the forecast is approximate.
- Ovulation day = cycle length − 14, never earlier than cycle day 8.
- Fertile window = the 5 days before the estimated ovulation day plus that day: 6 days.
- Approximate — when fewer than two gaps between periods are measured, when recent cycles differ in length more than usual, or when a gap falls outside the usual range. Such gaps stay out of the average.
- Late period — 1–2 days past the expected date the app says “Any day now”; from day 3, “Delay.”
- No guessing on stale data — after one cycle plus 14 days without a new period mark, the forecast stops.
Example: three measured cycles
Periods started July 3, August 1, September 1 and October 1: gaps of 29, 31 and 30 days, average 30. Ovulation is estimated on day 30 − 14 = 16, October 16, and the fertile window is October 11–16. The next period is expected October 31, so the card shows “Estimated period start Oct 29 – Nov 2.”
Example: one period marked
You chose “I don’t know my numbers” and marked one period starting October 1. Astras counts 28 days: ovulation on day 14 (October 14), fertile window October 9–14, next period around October 29. With no gap measured yet, the range widens to Oct 25 – Nov 2, and the calendar adds “One cycle marked — the more you mark, the sharper the forecast.” For a 21-day cycle, 21 − 14 gives day 7, so the estimate moves to day 8.
Ovulation calendar: an estimate, not a method
Astras doesn’t measure ovulation: there is no input for ovulation tests or basal temperature, only your marked dates, and every fertility label says so. Tap the estimated ovulation day and you see “Calculated from your period dates — not a confirmed ovulation.”; tap a fertile day — “Calculated from your period dates — actual fertile days can shift.” When today falls in the window, the week ribbon adds “A statistical estimate, not a confirmed ovulation,” and today’s card adds “Dates are an estimate — not a way to plan or prevent pregnancy.”
No day is labeled “safe” or “low chance”: an ordinary day outside the window shows only its cycle day. The disclaimer you accept at sign-up adds: “The cycle calendar is not a means of contraception.”
Menstrual cycle phases: a card for every day
Each day gets a card picked by your cycle day, not at random — from the first seven days of bleeding through the follicular phase, each day of the fertile window, ovulation and the luteal phase to the expected day and a delay.
- “In your body” — what usually happens in that phase, then what you may notice; the card doesn’t tell you how to feel.
- “Tip” — usually a fork, not a plan: if you have energy, one option; if not, another. No phase-based workout schedules.
- A source on every card — WHO, CDC, NHS, the U.S. Office on Women’s Health, ACOG or MedlinePlus.
- Facts beat forecasts — while bleeding is marked, you see only period cards.
- When dates can’t be pinned down — uneven measured cycles, or “Hormonal birth control” in settings — the cards for the fertile window and ovulation day are replaced with cards that explain why.
Cards don’t name conditions or make diagnoses.
Where your period data is stored and who sees it
Your cycle calendar is kept in your Astras account on our server, not only on your phone — that’s how it opens on a second phone or at astras.club, with marks from both devices merged.
- No ads in the app, and we don’t sell your data for money. Cycle entries never go to ad networks.
- Nothing about your health or cycle goes to advertising or analytics platforms. Ad analytics runs only after you allow it, in every country, and switches off in Settings.
- The calendar is health data and works only with your separate, explicit consent. Turn processing off and keep the data or delete it; export everything to a file or delete the account.
- Karina AI, the assistant in the app, is software, not a person, and can be wrong.
How to choose a period tracker: privacy, ads, honest forecasts
The “best period tracker” depends on what you need. Ask any app these questions:
| Question | Astras |
|---|---|
| Does it explain how the forecast is made? | Yes — the formulas above. |
| Does it admit uncertainty? | Date ranges, “approximate” on thin or uneven data, no forecast on stale data. |
| Where is the data, can you take it out? | In your account; export and deletion built in. |
| Ads, data sales? | No ads in the app; data not sold for money; cycle entries never go to ad networks; ad analytics only with your permission (California law counts it as “sharing” and a “sale”). |
| Is the calendar offered as birth control? | No. |
| Where does the health content come from? | Each card links to a public health authority. |
| Devices and languages | iPhone, Android, web; 8 interface languages. |
| Price | Paid plans from $4.99/month. |