Reverse Due Date Calculator
Work backwards from a due date to the estimated conception window and the date of your last period.
This runs the standard due-date rule in reverse: a due date is 280 days after the last period and about 266 days after conception. Handy when a scan gave you a date but you want to know roughly when it all started.
How it works
Sometimes you know the due date but want to know the rest of the timeline. Since a due date is set at 280 days after the last period, subtracting that gives you the last period date, and subtracting 266 days lands near conception.
The result is a conception window rather than a single day, because ovulation and fertilization do not happen on a fixed schedule. A spread of a few days on either side is normal and expected.
This is useful for curiosity, for lining up a timeline of events, or for double-checking dates from a different starting point. For medical decisions, always defer to a dating scan.
Frequently asked questions
How do you get conception from a due date?
A due date is 40 weeks after the last period, and conception happens about 38 weeks before the due date. Subtracting 266 days from the due date gives the likely conception day.
Is the conception date exact?
No — it is a window of roughly five days. Sperm survive for days and ovulation timing varies, so pinpointing one exact date is not realistic.
Why does this assume a 28-day cycle?
The standard due-date rule assumes ovulation around day 14 of a 28-day cycle. If yours differs, the conception estimate shifts by the same amount.