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Conception Date Calculator

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Enter your due date to estimate when conception likely occurred.

Conception Date Calculator

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Estimated conception date
April 24, 2026
Estimated last period (LMP)
April 10, 2026
An estimate based on typical cycle timing - actual conception can vary by several days depending on individual cycle length and ovulation timing.

The method

Estimated conception date = Due date - 266 days (38 weeks) Estimated last menstrual period (LMP) = Due date - 280 days (40 weeks)
Example

A due date of January 15, 2027: estimated conception ≈ April 24, 2026 (266 days earlier), with an estimated last period around April 10, 2026.

Step-by-step guide

  1. Enter your estimated or confirmed due date.
  2. Read the estimated conception date and estimated last menstrual period.

Why this is 266 days, not exactly 280

Pregnancy is conventionally dated from the first day of the last menstrual period (LMP) - a full 280 days (40 weeks) to the due date - even though actual fertilization (conception) doesn't happen on that first day. Ovulation, and therefore likely conception, typically occurs around two weeks after the start of the last period, which is why this calculator works back 266 days from the due date for conception specifically, rather than the full 280 days used for the LMP-based due date convention.

Common mistakes

Treating this as an exact, guaranteed date - actual ovulation timing varies between individuals and cycles, so the real conception date can fall several days before or after this estimate.
Assuming the due date itself is precise - due dates are estimates too, and actual delivery commonly falls within a window of about two weeks on either side.

Frequently asked questions

How accurate is this estimate?

It's a reasonable estimate based on a typical 28-day cycle with ovulation around day 14, but individual cycles vary - someone with a longer or shorter cycle may have conceived on a different day than this calculation suggests.

Why do due dates get calculated from the last period instead of conception?

The exact date of conception is often unknown, while the first day of the last period is usually a known, trackable date - this is why pregnancy dating conventionally starts from LMP rather than the harder-to-pinpoint conception date.

Can an ultrasound give a more accurate conception estimate?

An early ultrasound measuring fetal size can refine the estimated due date (and therefore this conception estimate) more precisely than LMP alone, especially for those with irregular cycles.

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