An estimate based on average cycle timing (266 days to birth, 280 from LMP) — individual results vary.
About this tool
The Conception Date Calculator works backward (or forward) to estimate when conception most likely happened, along with the fertile window and the pregnancy's key reference dates. Choose whether you are starting from a due date, a last menstrual period (LMP) or a birth date, pick that date, and the result updates instantly in your browser — nothing is uploaded.
All date arithmetic uses UTC day math so results never shift with time zones or daylight saving. From a due date, conception is estimated as due − 266 days; from an LMP, conception ≈ LMP + 14 days with a fertile window of LMP + 11 to LMP + 21 days; from a birth date, conception ≈ birth − 266 days. The tool also derives the related LMP (the day gestational age is counted from) and the estimated 40-week due date, since a standard pregnancy runs about 280 days from LMP and 266 days from conception.
Use it to narrow down a likely conception window rather than a single certain day — ovulation timing and cycle length vary from person to person. The output lists the estimated conception date, the likely window, the LMP reference and the due date.
Frequently asked questions
How is the conception date estimated?
Conception is taken as about 266 days before the due date (or birth date), or roughly 14 days after the last menstrual period. That reflects the average time from ovulation/fertilisation to birth in a typical 28-day cycle.
What is the fertile window shown?
Working from the LMP, the likely window spans LMP + 11 to LMP + 21 days, bracketing the estimated ovulation around day 14. Conception usually occurs somewhere in that band rather than on one exact day.
Why do due-date and LMP results differ slightly?
They use the same 280-day model but anchor it differently: an LMP counts forward 280 days to the due date and 14 to conception, while a due date counts back 266 days to conception, so rounding and your actual cycle length can nudge the numbers.
Are time zones a problem?
No. Every calculation uses Date.UTC-based whole-day arithmetic, so adding or subtracting days is exact and never off by one because of local time or daylight-saving changes. Still, the result is an estimate, not a certainty.
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