Family planning
Conception Date Calculator
Estimate a conception date by counting back 266 days from an established estimated due date.
Add source values for conception date
Entries used for conception date
The form asks for estimated due date because each has a defined role in this particular method. Copy measured or labeled values rather than estimating extra precision.
- Estimated due date: Enter estimated due date.
What is being calculated for conception date
Estimate a conception date by counting back 266 days from an established estimated due date.
Checking the conception date equation with an example
With the demonstration entries—Estimated due date 2027-03-21—the calculator evaluates the displayed equation. Replace every example value with values from the same case before relying on the arithmetic.
A useful conception date comparison holds every entry constant except the value being examined.
For a related but separate question, see pregnancy week and implantation estimate.
How the due-date back calculation works
The result panel reports estimated conception date. It does not silently supply a missing clinical value, unit conversion, or population assumption.
What conception date cannot establish
This is a conventional dating estimate, not proof of the date intercourse, fertilization, or implantation occurred. Official pregnancy dating may use other clinical information.
Record when the conception date inputs were obtained and which assumptions were selected before comparing another run.
Questions raised by conception date
What does the conception date result actually represent?
It represents estimated conception date produced by the due-date back calculation from the values entered on this page.
Why might another calculator give a different answer?
Differences in conception date often come from unit conventions, equation choice, reference data, or rounding rather than a malfunction.
Can this result make a health or treatment decision?
No. This is a conventional dating estimate, not proof of the date intercourse, fertilization, or implantation occurred. Official pregnancy dating may use other clinical information.
What should be recorded with the result?
Keep the date, all entered values, their units, and the named due-date back calculation method.
From calculation to record for conception date
Treat estimated conception date as one line in a record, not as a complete assessment. Add the date, circumstances, and reason for performing the calculation. A result without those details is difficult to reproduce and easy to misread.
The most useful follow-up is often another measurement made consistently, not another formula applied to the same uncertain input. When a decision has health consequences, confirm that due-date back calculation is the method that decision actually calls for.
Why the result is an estimate
Obstetric dating convention treats the due date as about 266 days after conception, while pregnancy age is commonly counted from an LMP-based point roughly two weeks earlier.
Biology and dating uncertainty prevent this result from proving a specific fertilization or intercourse date.
If the due date was revised after ultrasound or fertility treatment, use the date currently documented by the care team. Mixing an older due date with a newer clinical timeline creates an internally inconsistent estimate.