Conception Date Calculator
Estimate when conception may have occurred based on your estimated due date, with an approximate conception window to account for normal variation.
Calculate your estimated conception date
Enter your estimated due date to calculate an approximate conception date and conception window.
Your estimated result
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| Estimated due date | — |
| Estimated conception date | — |
| Approximate conception window | — |
| Estimated pregnancy length | — |
Step-by-step calculation
What does the result mean?
The estimated conception date is an approximate date calculated from the estimated due date. It should not be interpreted as the exact date conception occurred.
Pregnancy dating is based on an estimated timeline, and the actual date of fertilization can differ from the calculated date.
An estimated due date itself is not an exact prediction of when a baby will be born. Pregnancy normally lasts around 37 to 42 weeks from the first day of the last menstrual period.
How is conception calculated?
A typical pregnancy is estimated at about 40 weeks from the first day of the last menstrual period. Conception generally occurs about two weeks after that starting point.
This means the estimated time from conception to the due date is approximately 38 weeks, or 266 days.
This calculator therefore counts backward 266 days from the estimated due date to produce a central estimated conception date.
Because ovulation and fertilization can vary, the result is an estimate rather than a confirmed date.
Conception date example
Suppose the estimated due date is October 8, 2026.
Estimated conception date = October 8, 2026 − 266 days
Estimated conception date = January 15, 2026
The estimated conception date would therefore be approximately January 15, 2026.
Formula used
Approximate conception window = estimated conception date ± 5 days
The conception window is intentionally broader than the central estimate because the exact timing of ovulation and fertilization cannot normally be determined from a due date alone.
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Important: This calculator provides an approximate conception date for general informational purposes only. It cannot determine the exact date of conception, fertilization, or sexual intercourse. If accurate pregnancy dating is important for your medical care, discuss your dates with your healthcare professional. A dating scan may provide a more accurate assessment of gestational age.