- Next period estimate
- 2026-07-29
- Estimated ovulation
- 2026-07-15
Detailed breakdown
| Cycle | Next period | Ovulation estimate | Possible fertile window |
|---|---|---|---|
| 1 | 2026-07-29 | 2026-07-15 | 2026-07-10 to 2026-07-15 |
| 2 | 2026-08-26 | 2026-08-12 | 2026-08-07 to 2026-08-12 |
| 3 | 2026-09-23 | 2026-09-09 | 2026-09-04 to 2026-09-09 |
| 4 | 2026-10-21 | 2026-10-07 | 2026-10-02 to 2026-10-07 |
| 5 | 2026-11-18 | 2026-11-04 | 2026-10-30 to 2026-11-04 |
| 6 | 2026-12-16 | 2026-12-02 | 2026-11-27 to 2026-12-02 |
How it works
A window spans several days
The model includes five days before estimated ovulation and ovulation day itself. This six-day span is not six confirmed fertile days: uncertainty in the ovulation estimate shifts the whole calendar window.
Distinguishing a fertile window from pregnancy dating
A cycle-based fertile-window estimate and a pregnancy-dating calculation answer different questions. Use cycle inputs to explore a modeled window, or the appropriate dating mode when pregnancy information is already available. Calendar estimates do not confirm ovulation or conception. Avoid treating a calculated window as a reliable method of contraception or as evidence of a specific conception event.
Cycle-based fertile window: formula and assumptions
Next period = start + cycle length. Estimated ovulation = next period − luteal phase.
Cycle length varies. The fertile window shown is the five days before estimated ovulation plus ovulation day.
Last menstrual period: formula and assumptions
Estimated due date = LMP + 280 days + (cycle length−28).
Gestational age is conventionally counted from the last menstrual period. Ultrasound and clinical assessment can revise the estimate.
Conception date: formula and assumptions
Estimated due date = conception + 266 days.
A conception date is generally an estimate unless linked to assisted reproduction.
IVF transfer: formula and assumptions
Estimated due date = transfer date + 266 − embryo age days.
Confirm assisted-reproduction dates with your treating clinic.
Ultrasound dating: formula and assumptions
Estimated due date = scan date + (280−gestational days).
Use the gestational age supplied in the clinical ultrasound report.
Worked example
Using the Cycle-based fertile window example:
- First day of last menstrual period
- 2026-07-01
- Typical cycle length (days)
- 28
- Assumed luteal phase (days)
- 14
- Number of cycles
- 6
Next period = start + cycle length. Estimated ovulation = next period − luteal phase.
Example result
Next period estimate: 2026-07-29
Estimated ovulation: 2026-07-15
These are illustrative inputs. Change the values to explore a different scenario.
What to keep in mind
Dates are estimates, not a prediction of the exact day of delivery.
Calendar estimates do not confirm ovulation and should not be used alone to prevent pregnancy.
Formula references
References explain the underlying methods; they do not imply endorsement or individual professional review of Finflo.
Common questions
How is the estimated fertile window positioned?
The cycle model estimates ovulation from the next period and luteal phase, then includes the five days before ovulation and that day.
Can the calendar window confirm ovulation or prevent pregnancy?
No. Cycle timing varies, and the estimate does not confirm ovulation. Do not use it alone to prevent pregnancy.
Prepared by Finflo. See our methodology and editorial policy. Read the relevant professional-advice limitations.