How Conception Date Is Estimated
Conception typically occurs 11–21 days after the first day of the last menstrual period (LMP) in a standard 28-day cycle, with ovulation around day 14. To estimate conception date from a known due date, subtract 266 days (38 weeks) — the typical length of pregnancy from fertilisation. From the LMP, add 14 days (ovulation day). However, sperm can survive 3–5 days in the reproductive tract, and the egg is viable for 12–24 hours, so the actual conception window spans several days.
These are estimates, not certainties. Irregular menstrual cycles, delayed ovulation, and variations in individual pregnancies mean conception date estimates carry an uncertainty of ±5–7 days in typical cases. Ultrasound dating (usually performed at 8–12 weeks) provides a more accurate gestational age based on foetal size. The conception date calculator is useful for family planning, paternity estimation, and understanding the timing of early foetal development milestones.
Conception Date Reference
| Known Information | How to Estimate Conception |
|---|---|
| Due date | Due date − 266 days |
| LMP date | LMP + 14 days (standard cycle) |
| IVF transfer date | Transfer date (embryo already fertilised) |
| Ultrasound (8–12 wks) | Adjust based on measured foetal age |
