How it works
The calculator projects fertile windows and estimated ovulation dates across upcoming cycles, then shows a possible due date if conception occurred near the projected ovulation date.
Input query strings and outputs
Use an input name in this page’s URL as ?name=value, and join additional inputs with &. Portable shared links may instead use the compact ?ac= state parameter.
Input query strings
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?lastPeriodDate=First day of last periodDate · Required · Default: 2026-08-01
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?cycleLength=Average cycle lengthNumber · Required · Default: 28
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?lutealPhase=Luteal phase lengthNumber · Required · Default: 14
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?cycles=Cycles to planNumber · Required · Default: 6
Outputs
4-
fertileWindowNext fertile windowText · Primary output
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bestDaysHighest-probability daysText
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ovulationDateEstimated ovulationDate
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possibleDueDatePossible due dateDate
Variability
Even regular cycles vary. Fertility concerns or questions about timing should be discussed with a qualified healthcare professional.