How it works
A conventional pregnancy timeline places conception about 266 days before the estimated due date. The calculator derives the same point from several dating inputs.
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: due · Accepted values: due, lmp, conception, ultrasound, ivf
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?dueDate=Known due dateDate · Required · Default: 2027-03-08 · Used when method = due
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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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conceptionDateResultEstimated conception dateDate · Primary output
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possibleWindowPossible intercourse windowText
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dueDateResultEstimated due dateDate
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lmpDateResultEstimated LMP dateDate
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methodResultMethod usedText
Why a window is shown
Biology and calendar dating are not precise enough to identify one intercourse date in most naturally conceived pregnancies.