Reverse Due Date Calculator
Calculate the estimated date of conception or start of last menstrual period from a known due date.
Published by:
isCalculator Editorial Team
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How to use this tool?
How to use the Reverse Due Date Calculator:
- Enter the due date – Select the expected due date using the date picker. This is the only required field.
- Adjust cycle length (optional) – The default is 28 days. Change it if your average menstrual cycle differs (range: 20–45 days).
- Adjust luteal phase length (optional) – The default is 14 days. This is the time between ovulation and your next period (range: 10–16 days).
- Click "Calculate" – The tool will instantly show:
- Conception Date – Estimated day of fertilization.
- Last Menstrual Period (LMP) – Start date of your last period.
- Gestational Age – Always 40 weeks at the due date.
Tip: All inputs use days and dates only. No special formats needed. If you see an error, check that the due date is valid and that the luteal phase is shorter than the cycle length.
Previous Results
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Frequently Asked Questions
It calculates the estimated date of conception and last menstrual period (LMP) based on a known due date, using standard pregnancy length and cycle adjustments.
It provides an estimate based on averages. Actual conception and LMP can vary due to cycle irregularity and other factors.
These parameters adjust the calculation for individual menstrual cycle variations, improving accuracy.
