Reverse Due Date Calculator
Know your due date? Work backward to find your estimated conception date and last menstrual period.
Reverse Due Date Calculator
Can be past, present, or future
How the Reverse Due Date Calculator Works
Most pregnancy calculators take your last period or conception date and project forward to find your due date. Our reverse due date calculator flips this around — you enter your estimated due date (EDD) and it works backward to estimate when you conceived and when your last menstrual period likely was.
The formulas are simple inversions of standard pregnancy dating:
LMP = Due Date − 280 days
Conception Date = Due Date − 266 days
Practical Uses
There are several reasons you might want to use a reverse pregnancy calculator. Some parents want to know roughly when conception occurred — whether for emotional or practical reasons. Others use it to plan a target birth month: by entering a desired due date, you can see which conception window to aim for. Healthcare providers sometimes use reverse calculation to cross-check reported LMP dates against ultrasound findings.
Understanding the Conception Window
Our calculator shows a conception window of approximately ±3 days around the estimated conception date. This reflects biological reality: sperm can survive up to 5 days in the female reproductive tract, and the egg is viable for about 24 hours. So the actual intercourse that led to conception could have been up to 5 days before ovulation.
Remember that all pregnancy dating — whether forward or reverse — is statistical. Your actual due date depends on when you ovulated, when fertilization occurred, and individual biological variation. Use this reverse pregnancy due date calculator as an informative estimate, not a definitive answer.
Frequently Asked Questions
Common questions answered by our medical team