🌸Conception Date Calculator
The conception date calculator works backwards from the estimated due date to estimate when conception occurred. It subtracts 266 days (38 weeks) from the due date, as fertilisation to birth averages 266 days. The last menstrual period is estimated by subtracting 280 days (40 weeks) from the due date.
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Tip: Due dates from ultrasound (especially in the first trimester) are typically more accurate than LMP-based calculations, as cycle length varies and ovulation timing differs between women.
- 1Estimated conception = Due date − 266 days
- 2Estimated LMP = Due date − 280 days
- 3Gestational age from LMP = 40 weeks; from conception = 38 weeks
- 4Conception is typically 2 weeks after LMP in a 28-day cycle
Due date: 25 December=Estimated conception: ~1 April (266 days before)Estimated LMP: ~18 March
Due date: 1 June=Estimated conception: ~8 August (prior year)LMP ≈ 25 July prior year
| Calculation | Formula | Notes |
|---|---|---|
| Conception from due date | Due date − 266 days | Fertilisation to birth average |
| LMP from due date | Due date − 280 days | Standard Naegele calculation |
| Due date from conception | Conception + 266 days | More precise if ovulation date known |
| Due date from LMP | LMP + 280 days | Standard clinical calculation |
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