How this calculator works
The 266-day conception-to-due-date convention corresponds to 38 weeks, compared with 40 weeks from LMP. It assumes the usual 28-day-cycle equivalence. A birth date gives a much less certain estimate if delivery was early or late. Neither mode identifies the date of intercourse, establishes parentage or replaces ultrasound dating. Pending mode requires a future due date relative to the assessment date; historical mode accepts an actual birth date. The result is an estimate for information only and is not medical advice.
Formula
Approximate conception date = estimated due date - 266 calendar days. The historical birth-date mode uses the same full-term assumption.
Worked example
An estimated due date of 2026-10-15 gives approximately 2026-01-22. A full-term birth on 2026-10-20 gives approximately 2026-01-27.
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Frequently asked questions
Can it identify the day of intercourse?
No. Fertilisation timing and sperm survival leave several days of uncertainty.
Why are there two modes?
A pending pregnancy needs a future due date. Historical estimates accept past birth dates but depend strongly on the duration of pregnancy.
Does a leap year change the estimate?
The calculation subtracts 266 calendar days, so February 29 is counted when it falls in that interval. No separate year-length adjustment is made.