Pink Mom MathReverse Due Date Calculator
Enter a birth date to estimate when conception occurred and the last menstrual period.
How the Reverse Due Date Calculator Works
This calculator uses Naegele's rule in reverse — working backward from a birth date to estimate when conception occurred. Standard pregnancy dating adds 280 days to LMP to get an EDD; this calculator subtracts from the birth date instead.
The conception date is estimated as birth date minus 266 days (38 weeks). This accounts for the standard 38-week fertilization-to-delivery interval in a full-term singleton pregnancy.
The LMP estimate is birth date minus 280 days (40 weeks). This represents when the last menstrual period would have started, assuming a standard 28-day cycle with ovulation on day 14.
Important note: Actual conception timing varies. Ovulation can occur earlier or later than day 14, and fertilization may not happen on the day of intercourse. The estimate carries a natural uncertainty of approximately +/- 2 weeks.
Frequently Asked Questions
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This calculator provides estimates for educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your healthcare provider with questions about your health or your baby's health.
Based on Naegele's rule for pregnancy dating. Actual conception date may vary.