What this tool answers
The main example works backward from an estimated due date. It answers a different dating question from the cycle-based conception page and cannot establish the exact date of conception.
Formula
Estimated conception date = estimated due date − 266 days.
Worked example: Working backward from May 1
Using an estimated due date of May 1, 2027, subtracting 266 days gives August 8, 2026 as the illustrative conception estimate. The result remains approximate, even when the due date is entered precisely.
Load these worked-example inputs
Limitations
A due date and this derived date are estimates. The result does not establish parentage or identify a specific encounter. Clinical dating can use information not available to this calculator.
Frequently asked questions
Why is this due-date example calculated backward?
The estimated due date is the known input, so the stated dating interval is subtracted.
Does a precise due-date entry prove the conception day?
No. Precision in the entered calendar date does not eliminate biological or dating uncertainty.
Can this backward estimate establish parentage?
No. It is not evidence of parentage or of a specific encounter.