Reverse Due Date Calculator
Work backwards from a due date to when conception likely happened.
Estimated due date
Working backwards
A window, not a date
Standard dating puts the due date 280 days after the first day of the last period, so working backwards is straightforward arithmetic: subtract 280 days for the LMP, then add roughly 14 for likely ovulation. What you get is an estimate built on an estimate, since the due date itself already assumes a regular 28-day cycle.
Conception cannot be pinned to a day. Sperm survive in the reproductive tract for up to five days, so intercourse well before ovulation can result in conception on the day the egg is released. That is why the result here is a window opening about five days before likely ovulation rather than a single date — and why a precise conception date is not something any calculation can produce.
This matters most when the answer matters. A conception window from a due date is not reliable for determining paternity and carries no legal weight — the uncertainty spans roughly a week, and irregular cycles widen it further. Where the question is genuinely consequential, DNA testing is the only method that answers it. If the due date came from an early ultrasound rather than an LMP, the working-back is somewhat tighter but still a window.
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 window derived from a due date spans several days and is not reliable for determining paternity or for any legal purpose.Speak to a doctor or another qualified clinician about any health decision.Last reviewed .
Frequently asked questions
How accurate is a reverse due date calculation?
It gives a window of roughly a week rather than a date. The due date it works from is itself an estimate assuming a 28-day cycle, and sperm survival means conception can occur up to five days after intercourse.
Can this determine paternity?
No. The conception window spans several days and widens further with irregular cycles. Blood typing cannot answer it either. DNA testing is the only reliable method.
Why is the window five days wide?
Because sperm survive in the reproductive tract for up to five days. Intercourse well before ovulation can still result in conception on the day the egg is released, so the window opens before ovulation rather than on it.
Is it more accurate if the due date came from a scan?
Somewhat. An early ultrasound dates gestational age within a few days, which is tighter than LMP dating. The sperm survival window still applies, so the result is still a range.
What is the difference between conception date and gestational age?
Gestational age is counted from the last menstrual period, roughly two weeks before conception. So at the moment of conception you are already considered two weeks pregnant by the clinical convention.