Cycle length and ovulation timing vary, so an estimated conception date can be off by a week or more. Fertility treatments, such as IUI or IVF, produce known dates. The method assumes an average 266-day gestation from conception.
The result should not be used for legal or paternity questions.
Enter a known or estimated due date. The tool subtracts 266 days, 38 weeks, to estimate conception, and 280 days to estimate the start of your last period. If you have your ultrasound date, use the due date your provider gave you, since it is based on measurements of the embryo.
Conception can happen up to five days after intercourse because sperm can survive that long, so the estimate is a window, not a single day.
Conception Date: formula
Estimated conception date from estimated due date
| Estimated due date | November 23, 2026 |
|---|---|
| Estimated conception date | March 2, 2026 |
| Equivalent last-period start date | February 16, 2026 |
Common error: Using this estimate to establish paternity, legal timelines or any other purpose that requires a confirmed date rather than a population-level estimate. It reverses a standard obstetric dating convention, not a clinical or forensic determination of conception.
Conception Date: changing Estimated due date
| Estimated due date | Estimated conception date | Equivalent last-period start date |
|---|---|---|
| November 9, 2026 | February 16, 2026 | February 2, 2026 |
| November 16, 2026 | February 23, 2026 | February 9, 2026 |
| November 23, 2026 (example) | March 2, 2026 | February 16, 2026 |
| November 30, 2026 | March 9, 2026 | February 23, 2026 |
| December 7, 2026 | March 16, 2026 | March 2, 2026 |
Conception Date: individual input comparisons
| Input | Value | Estimated conception date | Equivalent last-period start date |
|---|---|---|---|
| Estimated due date | November 9, 2026 | February 16, 2026 | February 2, 2026 |
| November 16, 2026 | February 23, 2026 | February 9, 2026 | |
| November 23, 2026 | March 2, 2026 | February 16, 2026 | |
| November 30, 2026 | March 9, 2026 | February 23, 2026 | |
| December 7, 2026 | March 16, 2026 | March 2, 2026 |
Conception Date: values and definitions
| Quantity and definition | Value used |
|---|---|
| Estimated due date | November 23, 2026 |
| Estimated conception date | March 2, 2026 |
| Equivalent last-period start date | February 16, 2026 |
Working backward from a due date to a conception date uses the same 280-day, 266-day relationship Naegele's rule runs forward, which only holds as an estimate: it answers a different question than a due-date calculation, useful when the due date is the more confidently known figure, such as one confirmed by ultrasound, and the conception window is what's being asked about.
Treat the result as a several-day window centered on ovulation, not a single confirmed date, since natural variation in cycle length and ovulation timing means the true date can fall a few days to either side even when the due date itself is accurate.
Estimate the due date with the due date calculator and the current week with the pregnancy week page. The ovulation calculator shows the fertile window, and the IVF due date page handles transfer dates. Confirm dates with your provider's ultrasound.
Why is the conception date an estimate?
Because the timing of ovulation varies and sperm can survive for several days, so conception can occur over a window of several days.
How long after conception is the due date?
About 266 days, or 38 weeks, though doctors count 40 weeks from the start of the last menstrual period.
Is ultrasound dating more accurate?
Yes, especially in the first trimester, when embryo measurements are consistent. Providers rely on it more than on menstrual dates if they differ.