Sumyard

Conception estimate

See an approximate conception window from an estimated due date.

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Shows a broad ±7-day illustrative uncertainty window around conventional 266-day dating, not a validated confidence interval.

Your inputs

Updates as you type

approximate conception window

2026-08-08 to 2026-08-22

Central calendar estimate
2026-08-15

This approximate window cannot determine parentage or a specific intercourse date.

Calculations happen in your browser. Saving is optional.

How to use this tool

Enter the estimated due date you want to work backward from.

Read the central estimate and illustrative window together. Neither identifies an exact fertilization or intercourse date, or establishes paternity.

Approximate central conception date = estimated due date − 266 days

Worked example

2026-08-08 to 2026-08-22 · approximate conception window

Fixed example using the settings below. Monetary examples use USD; dates and times use their stated reference values. This example stays unchanged when you edit your calculation.

Example inputs
  • Estimated due date: 2027-05-08

Method illustration: A 2027-05-08 due date corresponds to a central conception estimate around 2026-08-15.

Assumptions and limits

  • Shows a broad ±7-day illustrative uncertainty window around conventional 266-day dating, not a validated confidence interval.
  • Cannot identify an exact fertilization date, intercourse date or paternity. Clinical dating and biological timing have uncertainty.

Method reference: NHS due date information

Check or report a result

Check the units, selected calculation and assumptions first. Compare the fixed example above with your inputs to identify what changed.

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