Conception date calculator
Use this to work out roughly when the pregnancy began, from the due date your doctor gave you or from your last period.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter i know my, the date i know and cycle length (last period option).
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
From the due date: conception = due date - 266 days. From the last period: conception = last period + cycle length - 14. The likely intercourse window runs from 5 days before to 1 day after, since sperm can live up to 5 days.
Example
Inputs: I know my = Due date, The date I know = 2026-10-08, Cycle length (last period option) = 28 days. Result: Likely conception date: Thursday, January 15, 2026 (2026-01-15); Likely intercourse window starts: Saturday, January 10, 2026 (2026-01-10); Likely intercourse window ends: Friday, January 16, 2026 (2026-01-16).
Limits and caveats
- The conception day is an estimate, usually off by a few days either way.
- It cannot establish paternity.
Questions about this tool
Why 266 days and not 280?
The 280 days run from the last period, and conception comes about 14 days later, leaving 266 days from conception to birth.
Can sex before ovulation lead to pregnancy?
Yes. Sperm can survive up to 5 days, so intercourse several days before ovulation can result in pregnancy.
Which is more accurate, due date or last period?
A due date from an early ultrasound is usually more accurate, especially if your cycles are irregular.