Reverse Due Date Calculator
Enter your due date to work backward and estimate when your last period started and when conception likely occurred.
Why work backward from a due date
A due date is normally projected forward from the last menstrual period, but sometimes it is useful to reverse the process: starting from a known or estimated due date to figure out roughly when the last period started or when conception likely happened.
How this calculator works
Enter your due date. The calculator subtracts 280 days (40 weeks), the standard length of a full-term pregnancy counted from the last menstrual period, to estimate when that period began. It also subtracts 266 days (38 weeks) to estimate a likely conception date, since conception typically occurs about two weeks after the start of the last period, and marks the approximate end of the first and second trimesters.
These are estimates based on population averages and assume a standard 28-day cycle; actual dates can vary. This calculator is for general educational and planning purposes only and does not replace dating confirmed by a healthcare provider or ultrasound.