Conception Date Calculator
Estimate when conception most likely happened from the due date or the first day of the last period.
Likely conception date
22 September 2026 Tuesday
- Likely window
- 19 September 2026 to 25 September 2026
- Last period started about
- 8 September 2026
Conception most likely happened around 22 September 2026, give or take a few days.
How it's worked out
- Conception
about 38 weeks (266 days) before the due date22 September 2026
- An estimate assuming a 28-day cycle. Early ultrasound dating is more accurate; ask your doctor or midwife.
How it works
Conception ≈ due date − 266 days ≈ first day of last period + 14 days
Worked example
A due date of 15 June 2027 puts conception around 22 September 2026, likely between 19 and 25 September.
Questions people ask
How do I work out the conception date from the due date?
Count back about 38 weeks (266 days). For a due date of 15 June 2027, conception was most likely around 22 September 2026.
How accurate is it?
It's an estimate within a few days, assuming a 28-day cycle. An early ultrasound dates a pregnancy more accurately.
Why is the due date 40 weeks if pregnancy is 38?
Due dates are counted from the first day of the last period, about two weeks before conception.
About this calculator
Formula and rules last checked 25 October 2026Written and tested by the CalculatorDex teamReport a problem