Conception Date Calculator
Estimate when conception may have happened from your due date, or estimate your due date from a known conception date.
This page is for general pregnancy planning and date guidance only. It is not medical advice, diagnosis, or treatment. Pregnancy dates are estimates and can change after a scan or advice from your midwife, GP, or maternity team.
If you have symptoms, concerns, or questions about your pregnancy, speak to your midwife, GP, or maternity team. For urgent help, use NHS 111; call 999 in an emergency. Read how we source and review content in our editorial policy.
Conception date calculator
Estimate conception date from a due date, or estimate a due date from a known conception date.
How conception date is estimated
A due date is often estimated as 40 weeks from the first day of the last menstrual period. Conception usually happens around two weeks after that point, so this calculator uses 266 days between conception and estimated due date.
Why this is only an estimate
Ovulation timing, cycle length, implantation timing, and dating scan measurements can all affect pregnancy dating. If your midwife or scan gives you an official due date, use that date for care planning.
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Sources and further reading
Checked for UK pregnancy dating guidance in July 2026.
We prioritise official NHS and GOV.UK guidance, then relevant professional-body information where needed.
Source links are checked on the review date shown above. Your midwife, GP, maternity team, fertility clinic, or employer can give advice that is more specific to your circumstances.