Conception Date Calculator
Estimated conception date from a due date or last period.
The uncertainty here spans a week or more. This must never be used to determine paternity — only DNA testing can do that.
Estimated conception
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How the Conception Date Calculator works
Conception happens within about 24 hours of ovulation, but because sperm survive up to five days, the date of intercourse and the date of conception are often not the same. Any estimate is a window rather than a single day.
Not medical advice. This calculator gives a general estimate, not medical advice. It cannot account for your individual health, medical history, or medication. Talk to a qualified healthcare professional before acting on any result.
Frequently asked questions
How is conception date estimated?
By subtracting 266 days from the estimated due date, or adding about 14 days to the first day of the last menstrual period on a 28-day cycle. Both assume ovulation happened on schedule, which is frequently not the case.
How accurate is a conception date?
It is an estimate with a window of several days either side. Even an early ultrasound, which dates a pregnancy more accurately than any calculation, has a margin of roughly ±5 days in the first trimester.
Can conception happen days after intercourse?
Yes, and it often does. Sperm can survive up to five days in fertile cervical mucus, so conception may occur several days after intercourse, whenever the egg is actually released.
Should this be used to determine paternity?
No. The uncertainty spans a week or more, which is easily enough to be misleading. DNA testing is the only reliable method, and legal or medical decisions should never rest on a calculated conception date.