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Pregnancy calculator

Estimate a due date and pregnancy weeks from the first day of your last period and usual cycle length.

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Answer

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Estimate a due date and pregnancy weeks from the first day of your last period and usual cycle length.

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Precision and calculation record
Rounding changes displayed numbers, not the calculation. Text, fractions, matrices and CSV schedules keep their own formatting. Automatic keeps up to 12 significant digits, with money shown to cents where applicable.
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Formula and assumptions
Calendar estimate for a regular cycle: last-period date plus 280 days, adjusted by cycle length minus 28. Pregnancy weeks here count directly from the last period; a clinician may revise dating after ultrasound. Irregular cycles, uncertain dates and assisted reproduction need other dating methods. This does not confirm pregnancy or predict a birth date.
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Using Pregnancy calculator
Estimate a due date and pregnancy weeks from the first day of your last period and usual cycle length.
Calendar estimate for a regular cycle: last-period date plus 280 days, adjusted by cycle length minus 28. Pregnancy weeks here count directly from the last period; a clinician may revise dating after ultrasound. Irregular cycles, uncertain dates and assisted reproduction need other dating methods. This does not confirm pregnancy or predict a birth date.
Next: Pregnancy weight gain calculator
Next: Period calculator
A worked example
For first day of last period (yyyy-mm-dd) = 2026-01-01, usual cycle length (days) = 28, date to calculate pregnancy weeks (yyyy-mm-dd) = 2026-02-12, estimated due date = 2026-10-08.
Before you use the result
Calendar estimate for a regular cycle: last-period date plus 280 days, adjusted by cycle length minus 28. Pregnancy weeks here count directly from the last period; a clinician may revise dating after ultrasound. Irregular cycles, uncertain dates and assisted reproduction need other dating methods. This does not confirm pregnancy or predict a birth date.
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ToolOctopus. “Pregnancy calculator.” Updated 2026-10-03. https://tooloctopus.com/pregnancy-calculator.
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