Reverse Due Date Calculator
Work backward from a known due date to estimate the original LMP and conception date.
What is Reverse Due Date?
If you know your due date but cannot remember the first day of your last menstrual period, this calculator works the math in reverse. It is most often used when due dates were set by ultrasound dating rather than LMP, or to satisfy curiosity about the conception window.
How is it calculated?
LMP = Due Date − 280 days. Estimated Conception Date = Due Date − 266 days. The conception window can extend 5 days before to 1 day after that estimate.
Frequently Asked Questions
- Why 280 days?
- 40 weeks of standard pregnancy duration.
- Why 266 days for conception?
- Pregnancy is dated from LMP; conception is ~14 days later.
- Is this exact?
- No — actual conception can vary 5+ days within the cycle.