Conception Date from Due Date

Estimated conception window = due date − 266 days, ± a few days for fertile window. Not medical advice.

Inputs

Result

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How to use this calculator

  • Enter due date.
  • Read estimated conception date and window.

About this calculator

Working backwards from a due date: subtract 266 days for estimated conception. The window extends 5 days earlier (sperm survival) through 1 day later (egg survival). Useful when paternity questions arise — but this is a formula tool only, not clinical or legal evidence.

Frequently asked

How wide is the conception window?+
Usually 6 days: 5 days before through ~1 day after ovulation. Conception is most likely on the ovulation day or 1-2 days before.
Is this useful for paternity?+
Only as a starting estimate. DNA testing is the only reliable paternity determination.
Can the due date be off?+
Yes — ultrasound dating is more accurate than LMP. The 266-day backwards calculation has the same uncertainty as the original due date estimate.
Medical advice?+
No. This is a formula tool. Pregnancy timing decisions should involve a qualified provider.
What if I had IVF?+
IVF dating is precise (count back from embryo-transfer plus implantation offset). For natural conception, the calculator's window is the best non-medical estimate.

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