Pregnancy Conception Calculator
Estimate probable conception dates and fertile windows based on your birth due date.
Based on standard 266-day gestation from fertilization.
Fertile Window Range
Trying to conceive rather than dating a pregnancy? The chance of pregnancy calculator estimates your odds this cycle from the days you had sex.
Medical disclaimer. This calculator is for general information and education only. It is not medical advice, not a diagnosis, and not a substitute for professional care. A conception date estimated from a due date or last period spans several days at best. It is not reliable for determining paternity and carries no legal weight.Speak to a doctor or another qualified clinician about any health decision.Last reviewed .
Frequently Asked Questions
How is conception date estimated from a due date?
Human pregnancy lasts approximately 266 days (38 weeks) from actual fertilization. Subtracting 266 days from your due date estimates the conception date.
What is the fertile window?
Sperm can live inside the female reproductive tract for up to 5 days. The fertile window spans 5 days prior to ovulation/conception through 1 day after.
How accurate is this conception calculation?
Unless conception occurred via IVF with known transfer dates, exact natural conception dates vary by 3–4 days due to cycle variations.
How is the conception date estimated?
By working back from the due date or forward from the last menstrual period: conception typically occurs around 14 days after the LMP in a 28-day cycle, or roughly 266 days before the due date. Because ovulation timing varies, the estimate carries several days of uncertainty either way.
How precisely can a conception date be known?
Rarely to the day. Working back 266 days from the due date gives the most likely date of fertilisation, but the due date itself carries uncertainty, and sperm can survive up to five days before fertilisation occurs. The realistic answer is a window of several days rather than a date.
Can this tell me who the father is?
No, and it should not be relied on for that. The fertile window spans about six days and the estimate carries several more days of uncertainty on top, so intercourse on distinctly different dates can easily fall inside the same plausible range. DNA testing is the only reliable answer to that question.
Why 266 days rather than 280?
280 days is measured from the last menstrual period; 266 is measured from fertilisation. The 14-day difference is the assumed gap between the start of a period and ovulation in a 28-day cycle. If your cycles run longer or shorter than 28 days, that gap shifts and the conception estimate moves with it.
Is the implantation date the same as the conception date?
No. Fertilisation happens in the fallopian tube within about a day of ovulation; implantation in the uterine wall follows six to twelve days later. Pregnancy tests detect hCG, which is only produced after implantation, which is why a test can read negative for several days after conception has already occurred.
Working backwards from a due date
conception ≈ due date − 266 days fertile window = ovulation − 5 days … ovulation + 1 day
The asymmetry of the fertile window is biological, not arbitrary. Sperm remain viable in fertile cervical mucus for up to five days, while the egg survives only 12 to 24 hours after release. That is why intercourse in the days before ovulation is far more likely to result in conception than intercourse after it, and why the window extends backwards rather than forwards.
Sources of uncertainty
Three compound here. The due date is itself an estimate unless it came from an early ultrasound. Ovulation timing varies by several days even in regular cycles, and stress, illness and travel shift it further. And gestation length varies naturally between pregnancies by a week or more. Treat the result as the centre of a range, and do not use it where a precise date genuinely matters.