How it works
The last-menstrual-period method starts from 280 days and adjusts for cycle length. Conception, ultrasound, and IVF modes use the corresponding developmental timing.
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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?method=Dating methodChoice · Required · Default: lmp · Accepted values: lmp, conception, ultrasound, ivf
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?lmpDate=First day of last menstrual periodDate · Required · Default: 2026-06-01 · Used when method = lmp
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?cycleLength=Average cycle lengthNumber · Required · Default: 28 · Used when method = lmp
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?conceptionDate=Known conception dateDate · Required · Default: 2026-06-15 · Used when method = conception
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?ultrasoundDate=Ultrasound dateDate · Required · Default: 2026-08-24 · Used when method = ultrasound
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?ultrasoundWeeks=Gestational age at ultrasound: weeksNumber · Required · Default: 12 · Used when method = ultrasound
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?ultrasoundDays=Additional daysNumber · Required · Default: 0 · Used when method = ultrasound
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?transferDate=Embryo transfer dateDate · Required · Default: 2026-06-20 · Used when method = ivf
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?embryoAgeDays=Embryo age at transferChoice · Required · Default: 5 · Accepted values: 3, 5, 6 · Used when method = ivf
Outputs
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dueDateResultEstimated due dateDate · Primary output
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conceptionDateResultEstimated conception dateDate
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lmpDateResultEstimated LMP dateDate
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earlyTermEarly-term date (37 weeks)Date
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lateTermLate-term date (41 weeks)Date
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methodResultMethod usedText
Date range
The result includes early-, full-, late-, and postterm landmarks for context. It does not predict the actual day of birth.