Conception Calculator

If you know roughly when conception happened, your estimated due date is 266 days later. If you know the due date, this page also runs the calculation backwards.

The standard due-date count (280 days from the last period) quietly assumes conception around day 14. Anchoring on conception itself removes that assumption — which is why this method is often preferred after ovulation tracking or fertility treatment.

Conception date (best estimate)

Estimated due date

Estimated conception

Implied last period (day 1)

Fertilisation to birth266 days (38 weeks), on average

Please read: This calculator gives estimates only — it is not medical advice and cannot diagnose or date a pregnancy with certainty. Only around 5% of babies arrive on their estimated due date, and most arrive within a week or so either side. Ultrasound dating performed by a clinician takes precedence over any calculator result. Always confirm your dates, screening windows and care plan with your healthcare provider. All dates are processed locally in your browser; nothing you enter is uploaded or stored.

Why 266 days?

Pregnancy length is conventionally counted from the last period — 280 days — because that date used to be the only one anyone knew. Embryonic development, though, starts at fertilisation, which typically occurs near ovulation, about 14 days after the period starts in a textbook 28-day cycle. 280 minus 14 gives the 266-day figure used here.

Conception itself is a window, not a moment: sperm may survive several days, so the day of intercourse and the day of fertilisation can differ. Treat any single "conception date" as the centre of a range of a few days, and remember that an early ultrasound, if your provider orders one, usually refines the estimate further.