Enter your due date to work backward and estimate your likely date of conception, along with a conception window and the corresponding last menstrual period date.
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This calculator works backward from your due date using the standard 266-day gestation period counted from conception (as opposed to the 280 days counted from your last menstrual period, which includes about two extra weeks before ovulation). It also shows a small conception window around that date, and the corresponding last menstrual period date for reference.
For example, for a due date of March 15, 2027: the likely conception date is estimated at June 22, 2026, with a conception window of June 20-24, 2026, and a corresponding last menstrual period date of June 8, 2026 (280 days before the due date).
The Pregnancy Due Date Calculator works forward, from your last period to your due date. This calculator works in the opposite direction, from a known or expected due date back to an estimated conception date, useful when you know your due date but want to estimate when conception likely occurred.
Treating the estimated conception date as a single precise day, ovulation timing naturally varies, which is why this calculator shows a window rather than one fixed date.
Using an actual birth date instead of a due date without accounting for early or late delivery, since babies aren't always born exactly on their due date, the reverse estimate shifts accordingly.
Confusing the 266-day (from conception) and 280-day (from LMP) gestation conventions, mixing them up produces dates that are about two weeks off from what's intended.
Conception can't be pinpointed to an exact day from a due date alone, since ovulation timing varies. The window reflects a few days of typical uncertainty around the calculated estimate.
Yes, you can enter the actual birth date in place of a due date for a similar backward estimate, keeping in mind that babies aren't always born exactly on their due date.
This calculator works backward from a due date to estimate conception. The Pregnancy Due Date Calculator works forward from your last period to estimate a due date. They're inverse calculations.
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