Estimate a conception date from the last menstrual period or a due date, with the fertile window around ovulation.
Result
How to use this tool
Use LMP if you know it, or a scan-based due date if you have one.
Only LMP mode uses it; the default is 28 days.
It is the ovulation day, not a proven single day.
Conception can occur on any day sperm survive before ovulation.
Formula & worked example
LMP 1 January with a 28-day cycle gives ovulation and estimated conception on 15 January, a fertile window of 10-15 January, and a due date of 8 October. A due date of 8 October gives an estimated conception of 15 January.
Limitations — read this before you rely on it
Educational aid only, estimating conception from dates. It is not medical advice and not proof of conception timing. Early pregnancy ultrasound dating is more accurate; discuss dating questions with a clinician or midwife.
Frequently asked questions
It is an estimate. Ovulation varies, sperm survive days, and early ultrasound dating is more precise.
Ovulation typically falls about 14 days before the next period, so cycle length minus 14 estimates it from LMP.
The 5 days before ovulation plus ovulation day, when conception is possible.
Pregnancy lasts about 266 days from conception, versus 280 days from LMP.
No. It gives a window, not proof, and should not be used as legal or paternity evidence.